I was actually thinking about doing this, only rather than a vertical pit, I was going to make an inverted pyramid with ramps leading between levels
I did that once, when goblins attacked, they shot from the edges all the way down to dwarves at the bottom...
oh, my intent was to have it still be closed off from the outside world (maybe extend the 'base' (top level) of the pyramid 1 level above ground onto walls with fortifications, and have a normal entranceway somewhere
like dis
The fort I made last week was a bit like that, except I carved a tower out of the center as I ramped down. I overdid it a bit, the rim of the pit was 40+ tiles away from where I had cut fortifications so my marksdwarves could shoot invaders as they streamed toward the tower entrance from all sides. This worked fine when it was ramped all the way around, but when I removed the ramps from the top so that the invaders had to walk all around the perimeter, they were too far away to get shot.
Just as well actually, even a champion marksdwarf shooting far away from the east at an enemy that's traveling north or south will just waste bolts (unless there's a whole line of goblins behind them and they're shooting at the first one).
Removing all that ground to make the pit with ramps was terribly laggy constantly, if I did it all over again I'd clear out the bulk of the area with just regular digging, then separate it from the surrounding supporting land with a ramped perimeter or channels from above (and a support linked to a lever somewhere for safety so my miners don't get squashed).
Also I feel a little silly because I realize the map I just started on already has a natural canyon and a few tall hills that could've been sculpted into natural towers, so I could've built a trade depot in the tower sculpted from the canyon and living towers in the mountains, in half the time it took me to sculpt just one out of a mountain. In fact I could still do that, except that the large-sized map with a bunch of chasm, a cave, a cave river and magma is already lagging me to hell and back.