I usually try attaching themes to my forts and base the architecture off that.
This one was intended as an epic mountainhome - a 26-story structure that is undeniably a
fortress literally carved into a mountain, guarded by a 'moat' that's more like a quarry (4-5 story drop with a curving 3-4 tile wide pathway leading to the entrance with an archery tower on either side), a courtyard filled with statuary depicting the history from around the world and the locally worshipped gods, an interiro chapel with carefully arranged and heavily decorated statues (one adamantine statue of the local god of scholarship has a value over 100k), three pyramids outside (one large with a 20x20 base, the others with 10x10 bases), a castle-like structure three stories tall with a footprint of maybe 50 tiles square, and a circular shrine-thing with silver-lined reflecting pools.
One of my other sites is in my sig, which should be pretty self explanatory: it's a huge damn wall.
Sometimes I just build a small outpost with a trading depot and maybe 20 beds before either abandoning or deliberately letting it get wiped out by goblins for the sake of including an interesting site in the world.
My current fort is going to turn into a sprawling aboveground mega-fort with different sections added in distinct bits. The initial fort is a 3 story square made of bauxite, the second addition is a jet construction that got tacked on later. I think I'm going to (in no real order) add a wing made from diorite, a large hall made of gabbro, a tall marble tower, a short courtyard made of microline or gypsum, and then maybe a marble wall surrounding the whole shebang just to prove that I can.