I got something new for today: Cannons.
Similar to dwarven turrets, but with more BAM! behind it. They are a immobile pet creature that you have to buy/construct. They shoot 85!!! tiles far, and the cannon ball explodes, instakilling anything it hits. But they have to be loaded manually, at an arsenal.
I added two workshops for this, the Arsenal E-W and the Arsenal N-S. They are 5x2 and 2x5, and should fit on top of your walls. The cannons can shoot through fortifications and across z levels, so thats fine.
Each arsenal can load multiple cannons, on the screenshot you see 2 on each. The cannons shoot up to 10 times for a period of 5000 ticks, which is about 1 ingame week, with 500 ticks delay between each shot. The shots shouldnt start any fires, but leave ashes and scorched earth where they hit. It shoots on wildlife as well, depending on type (I think benign creatures are fine, but in my test it shot at snakes.)
Loading the cannons when the enemy is right at your walls is difficult, because the civilian worker runs, but the setup on the screenshot works quite well. The work tile is in the center, while the fortifications are on the side, so if the worker gets to the arsenal, he will not see through the fortifications. You can also use raisable bridges to block Line-of-Sight.
Cannons are easily killed in melee, while surviving a few arrows, about 3-4, before they "die", leaving a damaged cannon. This can be repaired for a new cannon.
They are made at the Cannon Engineer, the final upgrade of the Engineers Guild. I havent written that part yet, but the cannons themselves work fine.
You can build them yourself, or order them from caravans, or buy them at embark.
I might do different ammo types later, for example one that does start fires, or one with warpstone in it.