A vault is a fortified structure used to store pearled players and/or other valuables. Pearls are typically held inside a chest, encased by a mound of obsidian at the center called a pyramid. These pyramids are most commonly found in large holes in the ground that have been dug out from surface level all the way to bedrock level. Other fortifications are often built to protect the pyramid, such as concentric rings of bastions and obsidian walls. Bunkers or towers are often constructed above the main part of the vault to store materials, factories, and potions for vault defense and to supply fighters.
A large ringed vault is often referred to as a "megavault", "gigavault" or even "hypervault" depending on the size. Ringed vaults generally take many hours to break, sieges against the largest vaults possibly even taking days. Vaults vary in size from the 4 ring former USA North vault to the largest vault on civclassic, Mir's Sanctaphrax, which contains over 30 rings.
Vaults have existed since CivCraft 1.0 after the PrisonPearl plugin was added. In those days, the original vaults were effectively just big obsidian pyramids located at bedrock level that would take hours to mine through to reach the pearl chest. As time went on and new mechanics like bastions were introduced, vaults got increasingly complex. The first ringed vaults emerged on Civcraft 2.0, attributed to individuals and groups such as FAGT, Peter, and Clone.
The first ringed vault to be tested in combat was Clone's vault which had only two rings, the outer being a conventional ring with reinforced web traps that did not follow bastion fields, and the inner being a lava ring designed to drown attackers. Although it has to be said this vault conceptually didn't represent a change in how vaults were defended; stopping attackers from reaching the pyramid at all wasn't the overriding goal. A short time later, the Titan Vault saw the logical progression of several features of Clone's vault on a larger scale. Reinforced web traps in the rings were devastating, and enabled the pearling of the person who went on to design the Titan Vault. Other increasingly antiquated concepts such as lava and water chambers were left behind as ring designs became more standardized across different schools of thought. Later on, more advanced vaults were pioneered by the World Police of the era during and after the fall of Titan. Some of those vault technicians included: Diet_Cola, TealNerd, and ShadedJon. The general bastion layout and ring shape designed by TealNerd around this time is also where the term "nerd rings" derives from. Diet_Cola iterated on the concept and created the infinitely scalable and consistent style of ring Bastion grid that became the standard for some years after.
Such large, advanced ringed vaults were thought to be unbreakable and became known as megavaults. This mindset would hold true for a long time, until the fall of the Nox Vault after a prolonged siege towards the end of the iteration.
During and after CivCraft 3.0, there was a significant schism in vault design. Diet_Cola pioneered a unique design now known as "cola cubes," while most others took the pre-existing template of nerd rings and optimized them far beyond previous standards. Cola cube vaults function as an array of bastion squares, while nerd rings have overlapping bastions in an octagonal layout, leading to high degrees of staggering. The schism in ring design furthered as the nation of Mir continued to develop linear bastion layouts for their rings, which are space-optimized and respect the horizontal boundaries of bastion fields, while cola cubes refused to respect either the horizontal or in-depth boundaries of bastion fields. For more information on bastion science and ring styles, see the section specifically covering rings below.
Over the next several iterations following the end of the CivCraft series, vault bastions would most-often retain their original circular shape, and vault designers primarily focused on optimizing their vault bunkers and sky defenses as ring science became more set-in-stone. This is largely because of the evolution of skybridging during the Devoted series as a tactic to break a path of bastions to the center of a vault. As where previously vault attacks had been almost entirely focused in assaults against an enemy vault's rings at bedrock level, now defenders had to be concerned with the heightened possibility of an attacking force breaking their bastions from the relative safety of any Y-level in the sky. This led to the increased development of multi-layered archer positions spanning from bedrock level to build limit, sometimes referred to as "archer pods" or "archer rings" that served as positions for defenders to shoot attackers off of their skybridges, making it immensely unsafe to skybridge an actively-defended vault.
After the release of CivClassic, Devoted 3.0 shut down, making CivClassic the new main iteration. In a rather atypical turn of events, the admin team of CivClassic decided to make both vault bastions and city bastions have a square shape. This led many vault designers like Frensin of Hjaltland to develop several different bastion layouts to fit square-shaped rings, while Mirian vault technicians like Lysika_Lantariel continued to perfect their linear layouts. Several other vault designers like those belonging to Lexington, the spiritual successor of Nox from CivCraft 2.0, continued to use a take on TealNerd's nerd rings, adapting them to suit the new square shape of bastions and creating very wide, jagged rings. Many aspects of vault design, beyond the shape of rings, stayed fairly similar to previous ideas from CivCraft and Devoted for quite a while.
However, throughout the Somber War between the Western Alliance and Lexington's alliance, a new form of skybridge pioneered by Frensin took the stage. A primitive initial draft of this new skybridge was seen at the first attack on the Anguish Vault, and by the final attack on the Lexington Vault, it had been perfected. The fully-developed form of the "Frensin Skybridge" as it became known includes a multi-layered obsidian box fully encasing and protecting attacking bastion-breakers, who are able to break bastions from the front of the bridge by standing against a wall of closed trapdoors and placing blocks against the front of a set of ladders. This style of skybridge is still the main tactic employed in a sky attack against an enemy vault, largely because of the relative safety and ease of maneuverability it provides a group of attackers, and because it has had an excellent record of successfully breaking to the center of almost every vault it has been employed against.
Largely as a result of the new advances in skybridging technology during the Somber War, many vault designers expended a much greater amount of effort developing more sturdy and elaborate sky defenses, as well as more clever bastion fields specifically designed to cause problems for a typical skybridge. Some notable developments in sky defense theory included denser walls of archer positions sometimes referred to as "archer rings" or "electric fences," as well as towers designed to enable defenders to counter-skybridge an attacking skybridge. However as of late 2020, many groups of players have yet to fully catch on to such methods of countering skybridges.
As of now, beyond mostly small differences between different schools of design and a few more closely guarded secrets, the meta behind vault design has largely stagnated, with years of development leading to relatively idealized defenses and tactics being implemented across most surviving vaults.
Notable Standing Vaults
Sanctaphrax is Mir's primary pearl-holding vault. It is a fully-entrenched hypervault located at the Eastern world border, boasting over 30 rings and several layers of sky defenses, surrounded by a large complex of defensive fortifications including XPLand. By number of rings and sheer effort put into its construction, Sanctaphrax is currently the largest vault in the history of the Civ Genre.
Wakanda is one of Mir's smaller outlying megavaults. Originally serving as the Anguish Vault in the Somber War, Wakanda was acquired and renamed by Mir after their annexation of the former nation of Anguish in 2018. It has been rebuilt and further fortified by Mir since then. Wakanda is located North of Sanctaphrax on the Eastern world border, is fully-entrenched, and currently boasts approximately 8-10 rings and sky defenses.
OGLand is one of Mir's smaller outlying megavaults. It was primarily built by TheOGFormula, a core member of the Mirian Council of Winners, sometime in 2019. It is fully-entrenched and currently boasts approximately 8-10 rings and sky defenses.
XPLand is not strictly a "vault" by most considerations. However, it is a fully-entrenched, densely-fortified, vault-bastioned complex on the Western perimeter of Sanctaphrax that often receives a sort of "honorable mention" level recognition as a vault in its own right. XPLand, as one could assume from its name, is chiefly used for the production of XP, accounting for the vast majority of Mir's industrial output of emerald blocks.
The HjaltVault/Hjault (Hjaltland)
The HjaltVault, located at approximately -3000, 11400, is a fully-entrenched megavault boasting approximately 10-12 rings and several layers of sky defenses. However, despite its modest size as far as megavaults are concerned, it is considered one of the strongest vaults on the server given the quality of its design. It currently serves as the main pearl-holding vault of Hjaltland, responsible for containing the pearls of over 100 hardened war criminals, petty raiders, and overly-curious newfriends alike, making its pyramid the most populated on the server! Construction of this vault began during the first week of CivClassic, making it the oldest surviving vault on the server.
Eternity is Hjaltland's only outlying vault. It is a fully-entrenched hypervault located at the Southern world border. Although the excavation of its massive 500 block by 200 block vault hole began sometime in 2018, Eternity sat incredibly unfinished for a very long time, only having 3, 4, or 5 rings depending on the direction you faced. Due to this fact, it was attacked and partially disabled during the Entente-Laconia War as part of the larger series of events that would later become known as the Infinity War. Despite being griefed, the vault was reclaimed by Hjaltland and the rest of the Entente shortly after the attack by the forces of Laconia, Corvus, and NATO, and was quickly expanded from there. Eternity and its exact ring count are once again greatly shrouded in mystery, and although known to remain unfinished, it is rumored that the vault is already much, much bigger than its older brother. Legend says it is planned such that one day, Eternity will grow so massive that it's trench will touch tips with the HjaltVault's trench.
The Crucible (Columbia)
A mid-size vault located near the Columbian town of Valyria, the Crucible boasts a respectable 8 rings and sky defenses, and currently serves as the main vault of the Empire of Columbia. The Crucible was an important site in the Columbian/Valyrian Theatre of the Infinity War, being used by the OkBomber Coalition to exert control in the deep -,- quadrant, and to push back the NATO forces of Polska and Hallow, eventually enabling the siege and subsequent destruction of the Hallow Vault.
A mid-size 6 ring vault in the sea east of NYC, located a few hundred blocks East of NYC.
Caledonia World border (Caledonia)
This doesn't exist.
Ice Box/FrostVault (Coventhia)
A large 9 ring fully trenched vault approximately 500 blocks west of Frostrock Valley, known as the "Ice Box", "FrostVault", or simply ⒹⒶ ⓋⒶⓊⓁⓉ.
Fort Briggs (Mount Augusta)
A large vault in the north east of Mount Augusta, consisting of about 10 rings. It is surrounded by a large trench. The vault is considered Mount Augusta's main vault, but is actually privately owned by citylion, a citizen of MtA.
Cringe Pit Home for Troubled Youth (Icenia)
The Cringe Pit is the main vault of the Republic of Icenia and sits east of Icenia City. This vault is complimented by the "Space Needle" a ringed skybunker located in the middle of downtown Icenia City, and Bloom Vault/Hyperion to the North.
A mid-size vault privately owned by Naglafer and Bonkill but subject to Icenian government oversight.
Dovrefjell (Imperial Truidence)
Dovrefjell, more commonly referred to simply as Dovre, is a medium sized vault located in central Imperial Truidence While still under construction, the finished vault features 8 rings, a trench, and dense sky defenses.
The Ark/Arx Centauri (Kallos)
The Ark or "ARX" as it is more commonly called is a moderately sized vault located in northern Kallos. This vault is still under heavy construction. The finished vault contains 7 rings, a PvP platform, trench, and a skyvault portion. ARX is one of the first vaults on CivClassic to utilize a unique sky-ring strategy as opposed to typical archer rings utilized in numerous other megavaults. A few minor details of this strategy is placing the ring walls at sky limit as opposed to bedrock, as well as creating underside archer positions to prevent acid-blocking.
Lexington's vault was located at North world border, has a 300 block radius trench surrounding it. This vault was the collective effort of a few Lexington players, but mostly taking influence from Aegis designers (most notably the ring layout). It was broken by SATO/Mir at the end of the Somber War, in the Fall of Lexington. It had 8 rings and two separate snitch curtains.
Admin Dsclouse took an album of screenshots of the vault attack, showcasing angles of an attack not normally player accessible.
The remains of the vault have been turned into a war museum and can be publicly visited.
USA North [Destroyed]
USA South [Abandoned]
HexCorp (Commonwealth) [Abandoned]
A private venture, left to decay after HexCorp CEO SerQwaez was pearled defending it. Located south of what is now MTA. Abandoned after several attacks from Bewsiej and other Lexington members. It has 5 rings/layers of cubes and is currently broken and bastionless after an alt raid from a banned player. Known as the "Iron Bank".
A 4 ring vault in a large hole, destroyed by Mir/SATO coalition on January 8th, 2019 after it had been abandoned.
In the summer of 2021, Krowatia began a restoration project of the land above and around the vault.
Heaven's Gate (Rhodesia) [Destroyed]
Was a vault named "Heaven's Gate" hidden by a massive trench, alongside a city, sitting at the South West world corner. Map data indicates it has 8-10 rings, with the outer rings in varying states of construction. The vault sits diagonally along world border. To the north and east cardinal there is a large skybunker before the trench. Similarly the corner of the trench has the same skybunker positioned just within. Disabled and destroyed by Mir/HJ/SATO coalition during the Mir-NATO war.
Sempiternal (Columbia) [Destroyed]
Sempiternal was a small 3 ring vault, built in the style of nerd rings. It consisted only of a ground bunker, sky bunker, and 3 rings, as well as a partially complete snitch curtain. It is currently disabled and abandoned due to the events of the Third Siege of Sempiternal by Mir, the Columbian Civil War/Columbian Bush War against Rhodesia, and further griefing by NATO forces, all in the course of events that would become known as the Infinity War. More images of Sempiternal before the war started can be found here
Hotel California (Latvian Soccer Dogs) [Destroyed]
Was a vault being made by ashnwill as part of the Latvian Soccer Dogs (LSD) alliance. This was destroyed and a by a joint force of Entente and SATO fighters on the 7th of September 2019 after ashnwill had quit without adding other LSD members to the vault groups, causing them to be unable to defend.
The Swimming Pool (Ransakistan) [Destroyed]
A small incomplete vault in eastern Imperial Truidence. The owners of the vault built it on Truidencian land, despite having no affiliations with the nation. It was disabled before the builders named it, so it was given the nickname "The Swimming Pool" by the attackers, the name derives from the flooding of the vault after the siege. The vault had 5 planned rings, with only two completed. It had a standard skybunker but lacked a snitch curtain. The attack on the vault happened in the wee hours of the 13th of December 2019 (UTC).
Bang City (Ransakistan/Corvus/Holy Jaded Empire) [Destroyed]
Mount Doom (Laconia) [Destroyed]
A large (? rings) vault in the +,+ quadrant built by Laconia. It was attacked and disabled by Mir and Entente during the Entente-Laconia War. It is known as "Mount Doom" from the name of the mountain that stood at the position of the vault prior to its construction.
Pinkerton (Holy Jaded Empire) [Destroyed]
While technically not a vault due to the lack of obsidian pyramid, this 5-ringed skybunker stands in Holy Jaded Empire territory, a few hundreds blocks north of Mount Augusta. The vault hole was originally dug by Laconian on secret alts in early-2018 but they were kicked out of the HJE following the Pinkertown Conflict. A proper vault was later built in the hole by the HJE. It was first temporarily disabled by Mir during the early stages of the Infinity war. NATO was however able to reclaim it and reinforce it. It was finally disabled by an attack by the Coalition on February 2020, exactly one year after the first capture.
A 1 ring vault/skybunker inside Corvus' trench, next to the town of Corvus. It was attacked while incomplete in early 2020 by the Coalition during the Infinity War and disabled.
The Hallow Pot Bunker (Hallow) [Destroyed]
A large vault near North West World border of roughly 29 rings. Hallow's vault's distinguishing feature is its snitch curtain. The curtain is densely packed for redundancy. The vault was first unsuccessfully attacked by Mir during the early-stages of the Infinity War during the first half of 2019.
It was later skybridged and destroyed by the the 'Coalition' on June 27th 2020.
The Best Western Hotel (Holy Jaded Empire) [Destroyed]
Located at west world border, and thus named the Best Western Hotel. It has 9 rings. An evolution of Lexington's vault design.
It was successfully attacked on July 18th 2020 by the Coalition during the Infinity War.
Vault 76 (Holy Jaded Empire) [Destroyed]
The Holy Jaded Empire's second vault. It stands 1500 blocks north of the Best Western Vault. It has 5 rings. It was successfully breached by the Coalition on July 18th 2020, during the Infinity War
The Litter Box (Hallow) [Destroyed]
A vault built in the NW world border of the map by NATO-affiliated players from the Geographica server.
Characteristics of a vault
A three part DevotedMC series, though somewhat outdated, offers a video explanation of foundational vault mechanics.
The vault pyramid is made of diamond reinforced obsidian blocks surrounding the chest to hold pearls. Pyramids/tall pyramids are the best way to store pearls as it can take quite some time to break through the blocks to reach the chest. Diamond reinforced beacons can also be used in the innermost layers of the pyramid as their break time is even higher than obsidian.
Pyramid have historically been designed to maximize the time needed to break them while minimizing the amount of resources (i.e: diamond) needed to build them.
This is especially important because the cost of a pyramid is
C(N) = 2*N3 + N , N being the height of the pyramid and therefore the number of blocks that will have to be broken in order to access to the chest at the center.
Adding an extra layer costs
Ca(N) = 2L(N+1)+1 
|Height||Additional Blocks||Total Blocks|
It is however possible to reduce the overall cost while keeping the same break time.
The first method is to build the vault on the world border. This divides the total cost by 2 as players can not go past the border and will be teleported back.
Another classic method to lower the cost was to optimize the slope. A steeper slope than 1:1 could indeed be more efficient as players were not able to fit in 1 block tall gaps and would have had to break more blocks if they were breaking a 1:1 pyramid from the side. Video explanation about vault pyramids
Nowadays, most vault pyramids are 1:1. The main reason is that most of the time spent during a vault break will be breaking bastions rather than breaking the pyramid. Moreover, by the time attackers reach the pyramid, the vault will be virtually lost and afking the pyramid break is not as risky as it used to be. Furthermore, players can now crawl and fit in 1 block tall gaps. Therefore, old break paths might not optimal anymore.
Rings prevent a player from walking over to a pyramid by being a physical wall they cannot break through easily. Ideally a player cannot place blocks to pillar over these rings due to bastion blocks.
Rings should be at least 6 blocks tall to prevent people from bypassing them with block placement or horses.
Shape and Bastions layouts
The most complete original ring vault was Titan where the bastion fields didn't follow the ring walls (or perhaps it was built incorrectly). The most important innovation since was to outline the bastion field perfectly with the ring wall. This was realised independently by Mir and Tealnerd during the aftermath of the Titan war. All subsequent vaults have followed this concept. It is effective as it means attackers have to be inside the ring, standing over the edge of the ring, or placing through an IRO hole they've broken in order to break the bastion all of which are either exceedingly dangerous or easy to counter.
Around Civcraft 3.0 bastion fields were changed from an approximation of a circle to a simple square. This caused some change in ring shape. Diet_Cola theorised Cola Cubes - fully walled bastion fields on all four faces with various traps within. Mir applied the simplest change - which was to have a square vault. Other groups, usually from the Aegis/Anguish school of vault building maintained the curved design of the rings despite having square bastions. Briefly the advantages are discussed below:
Cola Cubes - These are bastion efficient and excellent defence against a ground attack. They are very space inefficient. Few vaults use them now, however.
Mir Linear Rings - These are bastion efficient from the outside but are vulnerable to ring skipping. They are space optimised.
Rose Grid: A competing school of Bastion layout in Hjaltland, developed by SoulComplex and Servalclaw of Rosewall following the demise of Civcraft 2.0. Could maybe be described as a distant cousin of Mir's layout. At the beginning of CivClassic it was intended that Cola Cubes Vs. Rose Grid rivalry be reconciled by having two different vaults. However after the Lexington War ended the HjaltVault was rebuilt with a variant of the Rose Grid.
Curved Rings - These are neither bastion efficient or fully space optimised. It depends on the particular implementation. The advantage they do have is they provide an awkwardly shaped field for a skybridge to break. However, it is possible to determine the exact placement of the minimum number of bastions as each corner in the curve must be the corner of a bastion field. Effectively the classic nerd ring shape created by Diet_Cola during Civcraft 2.0.
Vaults outside the Lexington/Anguish/FRIENDS sphere of influence tend to use some take on linear rings. See: Coventhia, Yoahtl, etc. (Coventhia was originally a curved ring vault before Hjaltland shared some Bastion grid technology, and Yoahtl is a more primitive Rose Grid.)
While bastions layout are the most prominent characteristic of vault rings, rings will still have several other defensive features to improve the defense of the vault.
The ground is usually made out of a 2-block thick layer of obsidian. This makes griefing easier to revert as grief on the bedrock can not be removed using acid blocks but it also make space for traps.. These traps are often 2 block deep holes next to the walls. They can catch careless attackers who will find their mobility reduced. It is still possible to escape using knockback.
Rings are also laterally divided into several sections by dividers. These are meant to prevent attackers from having access to an entire ring when accessing one of the section. Dividers often have fence gates window in them to allow defenders to safely shoot the attackers from another section.
Some vault also have a carpeted ground. This serves several purposes: it makes griefing slightly easier to remove but also prevents mobs from spawning. It's also conveniently used to make the vault more aesthetic.
Some vaults will also have 1 or several tall rings to prevent players from pearling out. These rings are usually 21+ blocks tall.
A form of vault defense in the form of a floating obsidian bunker with bastions stacked below, typically situated above the vault pyramid itself. They often have built-in archer positions to help defend against an attack, and are a means of wealth storage as they are typically very difficult to get to, and break.
Sky bunkers can be accessed by using teleporters in the ground bunker below. These are often trapped to prevent intruders from easily accessing the sky bunker. Conversely, exiting sky bunkers can be done by using downward teleporters to the ground bunker. In order to prevent attackers from accessing the skybunker using newly built teleporters, it is strongly advised to avoid 4+ block tall spaces in it. This is because minecart teleporters will move the player to the highest block possible as long as there is a 4 block space above. This does not include space above sky limit.
They are furthermore protected by another set of bastions, usually in a cross-shape. This prevents players from being able to easily access it by pillaring from below as they will have to break each layer of the cross but this also increase the density of bastions against skybridges. Attackers who get close to it will indeed have to break at least 1 bastion per block before being able to access it.
They have a good location to store wealth. For that reason, it is a good location for expensive factories. Skybunkers should also have an area to brew potions.
Huge vault will also frequently have several smaller skybunkers in specific locations above the rings. These are either used to provide archer cover against ground attacks in the rings or as a means to guard against skybridges at sky limit.
Some sky bunkers, especially those from NATO circles will have dispenser aimed towards the outside. These can be used to fire potions or tnt towards skybriding attackers.
A bunker built around the DRO pyramid in the centre of the vault, used to store wealth and potion materials needed for defense. In most cases archer platforms are built into these bunkers and hold minecart elevators allowing people to get up into the sky bunker.
Ground bunkers tower above the rings and therefore usually feature archer position to safely shoot attackers in the rings.
They are also divided into several rooms. If one is breached then defenders can safely retreat to another one. These bunkers will often have small gaps to allow defenders to safely hit intruders from another room.
There is usually a small room right above the top of the pyramid that can be used to access the spike and store pearls.
A trench is much like a moat, combined with a vault ring. They are typically the first means of defense against attackers, and ideally are large enough to prevent scouts from getting close enough to the vault itself to prevent spying and world downloading. Some well developed moats would include the former Lexington vault trench, which featured archer platforms and a very large size.
Trenches usually have water at the bottom to allow players to safely jump into the trench.
They also have dividers to limit the mobility of an intruder along with traps to catch possible attackers.
Trenches were some of the very first Bastion defences theorised with designs conceptualised months before the plugin went live on Civcraft 2.0 in 2014.
Vaults will often have strong sky defenses against sky bridges attacks.
Sky "rings" can be built for that purpose. They feature archer positions and dividers. They are aligned over a ring to prevent people from breaking the bastion behind it. Skybridges have also been successfully defended against using dispensers and debuff potions (see First Siege of Sempiternal)
Some vaults have a buffer area between the rings and the trench. They have a lower density of bastions but still provide good cover for defenders against ground attacks.
A snitch curtain, or snitch grid, is a series of snitches surrounding a vault that make it impossible for someone to pass through without hitting a snitch and notifying the vault owners of an attackers presence. These should ideally be surrounded in Stone reinforced obsidian, to prevent the snitches from being broken by a TNT cannon.
There are several vault characteristics that have been tried in the past or theorized but aren't present on any major vault on CivClassic.
Instead of placing the vault pyramid at bedrock, it is instead placed at skylimit. This however causes several issues: the pyramid is not a pyramid anymore but rather a diamond. This doubles its cost. It is also now vulnerable to acid blocking.
Due to these issues, few major sky vault have ever been built. A notable recent sky vault is the Ascension Vault on CivRealms 2.0.
Some early post-bastions vault were immersed in water to slow down mining. In order to clear the water, players had to break the bastions first. At first the plugin worked in such a way that every single overlapping Bastion had to be broken individually which made combining a dense Bastion grid with a fish tank a viable gimmick. This design is now uncommon because it provides little benefit: the bastions that cover the pyramid will be broken anyway. Moreoever, when attackers who can start breaking the pyramid have usually secured the surrounding area. This means that increasing the break time of the pyramid would help defenders reclaim it.
Composites vault have several pyramids. This means that the owner can distribute pearls in these and that attackers will have to break all pyramids in order to free everyone. This can increase the break time without making the cost skyrockets as it is cheaper to build 2 10-layers pyramids than 1 20-layers one. Distributing the pearls also means that if a ground bunker falls then the defenders can retreat to another other one and continue the defense.
Elder guardians used to inflict the mining fatigue effect on CivClassic. This would have multiplied the time required to break a pyramid or at least required a significant effort to have attackers drink milk every 30 seconds. The Hallow vault was known to use this mechanics until mining fatigue was disabled by the administration team.
A thread containing vault pictures from CivClassic and prior to CivClassic can be found here