There is a labyrinth of caves and passages above the City that stretch up to the very surface. The Old Tunnels. An untamed place full of treasure and danger — the ruins of a civilization that had fallen millenia before the first game events took place.
The control over the dungeons is another important part of the game. The tunnel monsters are a constant threat to the City: they attack it one by one and in droves. By conquering the dungeons, not only does the player diminish the threat of those monsters, but also secures access to new resources and powers up the Guilds that would use the dungeons.
The tunnel system is generated anew whenever a new game begins. Every separate dungeon may belong to one of dozens kinds, including fortresses, catacombs, cave marshes, jungles, and many more. Each kind of dungeon demands an individual approach; the player would have to either upgrade their Guilds or spend resources to found the more relevant ones. Eg. Hunters’ Guild mercenaries are ideal if you’re surrounded by marshes, but they stand little chance against a well-defended fortress.
All dungeons in the player’s possession provide a modicum of resources for development. The Guilds also benefit from it, but in indirect ways. The warriors would use the captured fortresses as raid bases. The alchemists would make more efficient medicine out of the marsh herbs, and so on.
Dungeons aren’t a primary mechanic of the game, but by their very existence brings uncertainty into the City development progress, making the player think on their feet, planning upon the changing circumstances.