I embarked a fort at a goblin tower once and discovered a most wonderful landscape. The map had impassible cliffs completely dividing the lower goblin-inhabited half from the upper half aside from one six-tile-wide choke point that was right at my starting point, which I walled off immediately. I had a huge four-z-level-deep ravine in the area I'd claimed, and on top of that I had a brook at the highest elevation. It was perfect. I dammed the ravine, filled it with a gigantic lake of water, then dug a tunnel down into the basement of the goblin tower.
Did you know that huge amounts of high-pressure water squirting out the stairways of a goblin fort will actually form a pyramid of water over the structure, briefly?
Three legendary goblins drowned, one of them actually drowned while standing on the roof of his tower.
I just got started on a project to build a giant stepped pyramid in the desert that I think I'll stop and put off until after the save compatibility break. My plan is to dig away all the sand underneath so that the foundation of the pyramid rests on bedrock, and pump water up from the aquifer to cascade down the sides of the pyramid ala the hanging gardens of babylon. I've got the hang of penetrating aquifers, but doing it on such an enormous scale is a bit more of a challenge. Bring
lots of logs along (assuming you're in a desert with no vegetation, like I was).
A project I started but never completed was an attempt to build a fort that was completely surrounded by magma - all four sides, top, and bottom. Just stone pillar "stilts" holding it up. Wound up drastically underestimating how fast I could drain a magma pipe, next time I try it I'll build the whole thing in a separate cavern and then flood it with magma afterward.
Of course, I've also done an elaborate underground towercap farm. Always a good 'starter' megaproject. With lessons learned, my next one is going to be stupendous.