My hidden fortress with access only behind a waterfall.
That was back when I felt like there should be no trace of dwarfs on the surface, and this site proved perfect. Half the map consisted of a 15 z-level plateau. A brook ran over the top and spilled down a natural waterfall into a short valley, before running off the edge. I cut into the rock at the end of the valley, running parallel to it, then came out behind the waterfall, into a large mined out cavern that served as the entrance to the fort. Beyond that was going to be a very large, uninhabited, and richly decorated entrance hall that would serve as a point of defense. Then a straight run through a traditional fort to the King's quarters alongside a massive vault for riches.
Caravans would enter from on top of the plateau and couldn't interface with the fort itself. The brook would be dammed with a toggle switch that I was hoping could allow for an optional magma-fall. Otherwise, dwarf exposure to the surface, both living and constructed, would be minimal. There was even a very small tunnel leading from the starting temp fortress to the actual fortress, as a sort of back door escape route.
It just got too complicated for me after a while. Damming the brook was a long and painful affair. After that, I came under siege by a number of dragons. I waffled on designing the magma-fall (I was too afraid to try it). Finally, I got to work on the King's vault, but in the middle of that, I decided to utilize the construction space to attempt to cage an elf diplomat as part of an experiment to incite war. The experiment took too long and he went crazy. I burned out after that and the fortress is wallowing on my hard drive now, waiting for my attention.
Then another, the mountain peak fortress.
The fortress would be on top of a tall, steep mountain range, cut in half by a brook, leaving a shear drop between two peaks. Only access this time would be by crossing that drop on a narrow bridge from one peak to the other, which would be inaccessible from below. The outside would be all carvings and decorations on the path into the fortress.
It became a mess when I couldn't figure out the best way to get caravans inside through a separate entrance. It wasn't impossible, just more design work than I cared for. When it comes to too much uncertainty in design, I just back off. I took a long break from DF and when I came back, I felt like starting all over.
Now I've invested far too much time in Swordbaldness, even though I've already scuttled it once. I'm in this one for the long haul.