Hmm... let's see. The current fortress is well underground, except for the initial penetration fortress, which is all within the top two layers of soil. (Aquifer) There's an above ground wall protecting the trade depot, which will eventually go up another level and then be roofed over.
Down 15 or so z-levels into the stone is the bedroom complex, which is several groups of 3x5 bedrooms plus some 3x7's for those pesky nobles and officers and mayor types who want more than meager. Eventually they'll live down at -4z. (Magma is down at -7.)
And I'm currently building an Aquifer Flush Entrance, though it's a little slow going. The area is kind of tree heavy and getting a 5 by 5 square cleared takes some patience. Then I have to mine out the two levels to drop into the aquifer to punch through. It's only a 3 by 3 entrance I need, but it's not easy to get.
That won't get connected until I'm sure it's flood proof. Once is enough. Need another windmill though. And to get in and chop down that stupid tree.
Primary bridge, which leads to the Trade Depot has two over hanging archer lookouts, which will be able to shoot over the dry moat. (Eventually I'll have a water moat as well, but the Magma is too far away to to be any use. Pity.)
I've never been invaded before though. Never had a fort last long enough. We're into year two, so we'll see.
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I did at one point plan out a Tower Fort. One massive 30x30 King Tower, rising a majestic 40 z-levels, and four lesser Lord Towers, each rising 20 z-levels and containing central, spiral stair cases, filled with Weapon traps, and centers with fortified walls so my dwarf archers could shoot at the invaders as they attempted to ascend to the hanging bridges to cross into the King's Tower. The bridges of course only exist so that they can be dropped when the goblins are inside, falling 18z to the ground and being splatted by a de-constructing cave-in.