Before I start, you must know that I am a horrible trader. 6 times out of the 6 trade caravans arrived, I messed it up by either accidently offering wooden items to elves(making them leave instantly), or repeatedly making bad counteroffers to dwarves(making them say "Your childish games have made me tired" and leaving instantly).
I just got tired of making bad deals and when an elven caravan arrived, as soon as it started unloading their stuff, I annihilated the whole caravan along with the elves, their stuff, and the depot. I then ordered my squads to kill the surviving elves.
The result was 2 dead elves, 1 wounded dwarf(caught up in the cave-in when mining out the last tile), 1 dead dwarf(was hauling items from the caravan into my hospital during the cave-in, which is a known bug), some blood, and a big mess.
Now to mention that right above the depot, I had a room that I was planning to put a ballista in. It was partially destroyed while my miners were digging out walls to cause a cave-in.
When I wanted to fix the damage done to that room, I tried to place some walls. It said "needs walkable perimeter".
I thought, "You want a walkable perimeter? You get a walkable perimeter." and put up a floor at the spot where I wanted to have a wall. Then when the floor was built I tried to place a wall again, and it said "Building present".
Now I am a bit confused as to how to place a wall there without trouble.
Help?