Didn't read all responses (only first 5 on page 1) but in case nobody said it yet, the trick with towers is to build the corners first, and then the rest of the walls.
Keep in mind that only walls can support walls, so your tower can only get smaller to the top, unless you trick around with scaffolding.
A basic rule is to make the tower twice the height of any obstructions on your terrain. So if you whole map is +/- 1 z-level you want your archers to stand on z level +2 as 2*1=2. If your map has steep cliffs and hills and holes, then you can make a higher (+3 - +5) tower. Also make sure your tower has a roof so your archers are safe from flying attackers. Add in a hatch to the roof terrace, so your civilians can collect any arms/feet/heads that fly up onto your tower (sometimes shit gets lodged into the fortifications, too so you'll have to remove the constructions, and rebuild it, after the limb has been removed). If your tower isn't just a ground level fortification (and even if it is, just dig a hole under it), you can put archery targets and ammo stockpiles on the lower levels, for example ground level = arrows, first level = archery targets, 2nd level shooting range, 3rd level = roof. You can then just have your 5-10 marksdwarf in a squad and have them on inactive with all labours disabled, so they will train and once you're under attack, you put them on active with a schedule that says station at note and a note on the 2nd level of your tower.
Hope that makes sense.