In my latest fort, I wasn't going for a long run fort at all. I wanted to see how fast (in real time) I could get the starting 7 up to full automation for food, drink, clothes, etc. I had above ground crops because I set my population cap way below the siege threshold... And then I got sieged by the undead... I turtled for a bit and then found myself liking the fortress.
I ended up building a very small keep on the top with walls to enclose my garden. My underground fortress was going in to the side of the mountain and I decided I wanted some more space above ground. I excavated out a nice plateau and surrounded that with walls. So that meant that I'm now on 2 levels above ground. I kept getting sieged by the undead and just kept building and expanding when I could.
I put another level on and more walls and then thought I should put fortifications all along the walls -- hoping the Toady fixes the ammo bugs before I finish the fortress :-). I built out a large open area and put a floor in and then turned it into a barracks. When I'm sieged, they can go practice out in the yard with the undead milling around outside the walls! But, of course, rain.... So I built up some pillars and built out a roof, similar in style to a Japanese castle. And then, my hopefully-able-to-shoot-one-day crossbow dwarfs need a walkway all around the perimeter and a roof overhanging so they don't jump over the battlements (or get rained on).
Basically, at every level, I'm building out walkways, open spaces, roofs and connecting them all up. I've build out a pasture for the animals behind the castle and I'm thinking of building a big out door temple with a pagoda.
The interesting thing about this fortress is that none of it is planned. I just had a garden that was inaccessible to me and wanted to wall it in. Then I wanted more space, so I walled that in. Then I wanted more levels, so I walled that in. Then I wanted a place to train, so I built that up. Then I wanted access to the walls. Then I wanted roofs. Then I wanted a pasture. And now I want a temple. And each time, I'm building out as safely as I can, extending my walls, enclosing spaces and turtling from the enemy. And... it looks like a real castle -- probably because that's how real castles were build :-)