why do you build the wall so high?
A few reasons.
1) Invaders can climb, now. Level one walls now offer very little trouble to a determined attacker. There's a chance of slipping and falling for each level climbed, so a three-level wall is more difficult to climb than a one-level wall. I am also planning to add an outward overhang to the wall, so that climbing it won't work unless the invader can also crawl across the ceiling upside down to get past the overhang.
2) I'm going to put a row of fortifications (arrow slits) along the top of the wall, so Marksdwarves can line up along the wall and shoot at invaders in the fields outside. Not sure if you know how Fortifications are treated in-game, but the protection they offer depends on how close each party is to the fortification. If the enemy can get adjacent to the fortification, then it offers my dwarves no protection at all... and the further the enemy is away from it, the more protection it offers me. So putting my fortifications on top of a level-3 wall ensures that the opponent cannot get closer than three tiles (unless he can fly, of course).
3) When Marksdwarves are close to the enemy and run out of bolts, they have a distressing tendency to run into melee and start whacking opponents over the head with their crossbows. This rarely ends well. So I'm putting the fortifications up three levels, to keep my Marksdwarves further away from their targets, reducing the chance that they will pull a "Leroy Jenkins".
no moat? you are not afraid of Trolls that can destroy buildings? including walls/bridges and doors?
Building Destroyers cannot destroy walls. They can indeed destroy drawbridges... from all sides except the bottom (the outer face, when raised).
That means that it is ESSENTIAL to always counter-sink your drawbridges, so that when they are raised, the left and right edges disappear into the wall and only the (invulnerable) outer face is accessible.
Doors? Any doors that an opponent can reach will be gated behind a counter-sunk drawbridge immediately in front of them.