I've dug a sizable area out of our mines for a tree farm. Once complete, our underground will be entirely self sufficient in the unlikely event our bedmoat is overrun, our walls climbed, and our town pillaged.
Don't make the same mistake I made, which was to put torches on the walls. Also remember we want the ceiling so short that we can chop down the entire (preferably birch) tree without dirt scaffolding.
Oops, I kinda torch'd the wall.
Fortunately, the room is ENORMOUS, so trees should still be fully capable of growing.
Also, we need to do something about the caverns under our moat, especially since some of them lead right into our base. As is the case right now, we have exactly as much claimed land and moat as we need to be basically impossible to conquer (barring land claiming, which we can stop by being online often and not dying), but there are some caverns beyond our moat that clever raiders could dig in through to reach our valuables.