Aquarium: Build your fortress out of glass blocks on the ocean bottom. Main fortress may extend two levels below sea floor, but cannot pass under land. Mine shafts allowed anywhere, but no buildings/rooms except in the main fortress.
Battleship: Build a large checkerboard area around your fortress, at least 4x4, with a replenishing pool of magma beneath it. When enemies come onto the checkerboard, have a set of levers which can lock any cell of the checkerboard and then flood it with magma. Must be Troll-resistant.
Labyrinth: Build your fortress in the center of a huge maze. Make the walls shift, by means of bridges/floodgates, and have at least some of the shifts be automatic, preferably random. Traps are a nice bonus, though not necessary.
Skyscraper: No more than eight 3x3 rooms per floor around a central 3x3 staircase, entirely built above ground. The only exception to this is the ground floor, which is allowed to be 18x18 and filled with any kind of rooms you like, provided the 3x3 staircase is built. You may have multiple towers if the first one is entirely full and reaches as high as it can go, with the provision that none of the dwarves in one building are allowed in the other and vice versa, once the second tower has been built. You may not dig mine shafts of any sort, but you may dig out a large quarry at some distance from your towers and use the metal/gems you find in the quarry. Glass windows in every room are a bonus.
Tetris: Have a series of pumps/pressureplates/bridges/floodgates in your main entryway so that whenever intruders enter, the ground is flooded with water and Magma is dumped on top of them in the shape of a random tetris block. Simple in concept, ridiculously complex in execution.
Universal: Do all of the above at once. Which is to say, surround part of the ocean with a grid you can selectively pump magma into, then have it transfer to an underground shifting labyrinth which leads to an entrance hall with the tetris trap in it and then to the glass aquarium fortress, on top of which are a couple of giant skyscraper-style towers.
... I really want to try that last one now.