Expanding on the Sunken Ruins

These are my thoughts thus far on the Sinking Ruins, my prospective mega non-dungeon that I may write up and run someday. Or not - I've got a lot of Savage Tides to run for my tabletop group first, and that means a fair bit of time spent converting it to GURPS DF first. It's useful to stash these kind of thoughts, anyway.

History of the Sunken Ruins

The origins and the name of the Sunken City have been lost to any history outside of its own buried archives. Indeed, the very existence of the city was lost when it was razed during the 7th Crusade. The area was resettled by the villagers of Shady Shore, who had only begun to discover the ruins below their fields when the orcish Il-Khan's army dispersed them. The orcs did not linger, and after they passed, Blessedborn monks established the Monastery of the Sacred Victor on the site. Within three generations, the monks had fallen to demon worship, and Templars of the King of Life besieged and burnt the place. Ten years later, rumors have sprung up about the treasures hoarded by the monks and left undiscovered in hidden parts of the cellars, but nothing definite is known.

A Note on Climate

The Sunken City was built in a quite northern climate, and densely packed buildings, covered arcades, and sheltered market areas are a common feature of the local architecture. In addition, ever since the 1st City was buried in the Cataclysm of the Scarlet Warlocks, the ground has been too soft to keep buildings from sinking, and the locals tended to periodically re-elevate their roads and reroof their buildings as ground levels sunk so deep as to become basements.

Levels of the Sunken Ruins

• Surface Level: The Ruined Village of Shady Shore and the Abandoned Monastery of the Sacred Victor - half sunken and burnt buildings, collapsed and ruined. Inhabited mostly by bandits and savages. A few half-collapsed delving tunnels, wells, and purple worm tunnels provide access to the deeper levels, and stairs and ladders go the buildings' basements.
• Sublevels 1-2: Mostly the cellars and subbasements of the village and the monastery. A few collapsed cellar floors provide access to the upper levels of the 3rd City.
• Levels 1-3: The levels of the 3rd City, built over the previous two cities. Consists of buried and collapsed buildings, connected by the purple worm tunnels that riddle the area and break into buried arcades and buildings, as well as the ancient and often collapsed covered streets. On level 2, the Dwarven Delving Hall contains a wealth of information about the 2nd city, including maps. On level 3, the old city cistern can be accessed from the surface through some of the wells. The dwarven delving tunnels descend through the sewers into the 2nd City. 
• Level 4: The sewers of the 3rd city, ancient brick tunnels pierced by delving shafts and a few purple worm tunnels.
• Levels 5-7: The levels of the 2nd City. Notable sites include the Cathedral (a multi-level vault that acts as an effective elevator for delvers in the 2nd City) and the Castle (with passages inside the walls). In addition to the buried roads, there are plenty of dwarven delving tunnels branching out between buildings and heading toward the Castle.
• Sublevel 7: The hard cap created by Cataclysm of the Scarlet Warlocks, forming a cavern over much of the 1st City. A few tunnels, wells, and magical portals pierce it.
• Sublevel 8: The Adamantite Fortress, a castle made of essential stone. Technically part of the 1st City, but on a hill and above the main floor of the city.
• Level 8: The nearly intact 1st City in a cavern. A good place to put a dragon. A lot of orichalcum weapons are here (that's what the dwarves of the 3rd City were seeking - they just didn't know about the 2nd City and weren't going deep enough). High level playground. Think of Xak Tsaroth. 
• Level 9: Mines of the 1st city, accessible through adits in the 1st City.
• Level 10: The ancient sewers of the 1st city, mostly made from abandoned mines. Some mines go through or below the sewer level.
• Levels 11-12: Natural caves. The sewers and some of the mines enter directly into these.
• Level 13: Hell. "Dug too deep" shouldn't mean "oh, and we found a balrog." It should mean "no, there's a literal Hell in the underworld and you can get there by going deep enough." (Just like you should be able to get to Heaven or at least the moon by flying high enough. Fantasy physics should be fantastic.) As a bonus, any delvers that actually reach the 13th level are going to be 1000 point Lords of Crazy Town, so you might as well send ST40 DR10 Skill-26 demons against them as mooks so there's some sense of challenge.