Well, you can do what this guy says:
You can also build water traps, magma traps (for maximum dwarf), you can do what I do and make a corridor that alternates between three tiles wide and five tiles wide, and in the indents chain your most deadly beast of war (in my case the giant platypus), you can build ballistas, catapults, and even fortifications to allow your marksdwarves to shoot through without getting involved in melee, you can do what I was thinking about doing before and have a bridge over a small manmade swamp filled with platypus (or crocodiles, etc.) which are almost guaranteed to destroy your enemies if they are poor swimmers (especially the platypus as their venom can cause beasties to pass out from the sheer pain which then drowns them), aaaaaaand there are many more, but I'm tired as hell and can't think.
One last idea that I've never tried: water or magma cannons. Basically you use pressurised water or magma to shoot out and destroy your enemies.
This is why I love Dwarf Fortress.
Press b, then press a capital C, then select walls and build the tower. You can then add stairs from the same capital C menu.
A fun thing to do (which I've already mentioned in this thread) is chaining prisoners on towers to serve as lookouts whilst simultaneously punishing them.