Casement Rock (formerly known as the Inner Fort) is a small fortification in the middle of the harbor that protects the entrance to the city's waterways. The admiralty is, of course, the headquarters of the city's naval fleet. And the flooded tunnels are... well, they're whatever you want them to be. Here are a few ideas:
The tunnels have passages leading up to various places in the city. The entrances are located under flagstones in the floors and the buildings' occupants may not be aware of them.
The tunnels could be mines, filled with the undead corpses of the miners who died when they flooded.
Aquatic creatures occupy the tunnels. They may be hostile to the people of the city or they may not. These could be kuo-toa, sahuagin, merfolk, lizardmen, sea elves or an aboleth and its minions.
The tunnels could connect to the lair of a large sea creature such as a dragon turtle or a kraken. This creature might be controlled by someone in the city. Or it might be secretly controlling the city.
You could also just connect the tunnels to a dungeon, which could be located in an air pocket if you want.
Those are my ideas, anyway. If you have any other thoughts about what you might use the tunnels for, let me know.
I'll be coloring the palace next. And I might just get the mages' island done while I'm at it. After that, there's just the wealthy district and the grand market left. This thing is getting close to being done! I hope you like it so far!
Download from Google Drive here (under NEWEST PATRON CONTENT).