I've had a lot of experience with constructions in making my various above ground, and floating, forts. Some tips, don't build fortifications, but rather build walls and carve fortifications. This takes more time, but saves materials because you will still have the extra floor above it as though it were a normal wall, which you wouldn't have by just building fortifications.
Second, what was said above is very true: Build corners first. Though, dwarves CAN built diagonally, they will either refuse or be unable to if there are two walls near it making the only way to build there diagonally. I've had dwarves really piss me off when they go diagonally to build a wall or floor after they refuse to build the corner wall diagonally.
Third, if this is going to be a defensive tower then some ideas you may want to implement. Have the entrance to it be by a staircase accessible only from inside the fort, not a door on the ground level. Destroy all ramps to the upper part of the mountain near it so it's only going to be hit from one side. Build a wall with a 5 square wide bridge as a gate, so sieges will have trouble getting through but wagons and such still can. Try making a moat with a brudge instead of the wall and bridge mentioned above, if you can.
Fourth, from what I've heard you can't fire from one Z-level to another.