I just dealt with the first kobold thief. My two war dogs took care of him pretty easily (I also drafted my weapon-wielding miners and woodcutter in case) but it got me thinking about my military preparations.
I intend to create a moat over which a drawbridge will control entrance to my fortress. However, not being willing to channel down (because much of my fortress is on the first z-level), I want to simply set up all my defenses one level in the air, using a mass of constructed walls. I have hundreds (450+) of chalk pieces lying around so materials are not an issue.
So first of all, how effective is a constructed moat? That is, I have a ring of walls, and the only way into the walls is by means of a drawbridge, which connects to a wall outside the ring. This wall has ramps for caravans. I'm trying to avoid digging a moat because, as I said, most of my fortress is on zlevel 1.
Second, can I place traps on a bridge? If not where should I put them? At the base of the outside wall, ahead of the bridge, or at the base of the inside wall, once someone steps off of the bridge? And will my traps obstruct caravans?
Should I construct a platform so that ballistae can fire down the bridge?
Finally, can I construct walls on the bridge so people don't fall off, or will I just have to live with that risk?