It can be helpful to have your towers be fully self-contained as well. I've got a pair of them out past my main fortifications on my current fort complete with a barracks, dining hall, food stockpile, and archery range under the tower. Tower's connected to the fort by tunnels.
Towers are 2 z-levels high with forts on the top (good idea about the bridges in case of gobbo elites, will add those), and walls on the bottom. Ammo's on the ground floor, firing platforms on the top obviously, and all the other accoutrements down below ground. Surround the whole thing with a dry moat and you're in business. I've basically got a whole squad of marksdwarves living in each tower 24/7, all year round. Only open up the connection between the towers and the fort to refill the food stockpile. In the meantime, my soldiers are getting legendary meals and looking at nice statues to keep happy. Might even put in a mist generator in each tower for added luxury, but we'll have to see about that.
*EDIT* You can't build the down stair on the second level until the up stair on the first level is complete. Also have to make sure you're directly above it, or you'll get that message. For a roof on a 2 z-level tower, you need an up stair on ground level, then an up/down on Z+1, THEN a down stair on Z+2, then you can finally place the roof. I'd do this on the outside, easier to take down and just build a new staircase inside to get to the firing platform.