The Causeway.
Basically I've got a 5-wide, several hundred long road that extends to the end of the map from my trade depot. The depot is surrounded by four, 7x7 buildings that each contain a 5x5 farm plot. The road is separated from the fortress area by a retracting bridge. I dug through a couple hills for the road, and then got the idea to make the entire thing covered, from map edge to the bridge. But then I thought, "Why stop there?"
So I'm going to collapse the remaining area of the hills over the road, and wall the whole thing in, with the exception of a couple areas for wayward caravans to reach it (although so far all trade carvans have come from the road). Then I'll cover the entire thing, and start going upwards. Further, my four farming greenhouses will go up, and connect to one another (short distances mean there's plenty of support for bridging the gaps), and then... connect them to the Causeway one level up. The bridge will still be retractable, leaving any attackers with a 3-wide moat to deal with if they want to get at me, but meanwhile I'll be able to travel freely overhead. The Causeway will be multiple stories tall, and inaccessible except by first getting past the moat and then going upstairs in one of the four towers, then exiting the towers into the connecting platforms, and from there heading into one of the levels of the Causeway. I might also add some traps to the surroundings to help ensure the doom of sieges. One idea is to moat either side of the road, and then add a few retractable bridges so that I can completely isolate the entire structure if need be.
I might even install some fortifications and siege engines, just in case we get a compatible z-level shooting upgrade.