So I'm playing my latest fortress to have a better defense system, as the last few have fallen prey to raids and sieges leaving only one or two dwarves alive. My last fortress, I used the multiple entrance strategy, where in the second entrance was longer and had a bridge, and the quick entrance was trapped (to exploit goblin artificial stupidity). The problem was, that my stairwell was near the trade depot,which was in front of the main entrance and allowed goblin bowmen and crossbowmen to kill any wanderers who walked by.
This time, my plan is to build a trade depot on the surface, with a wall all around. On the north side, I plan to build a bridge for a gate. On either side of the gate, I plan to make a two level 'tower'. On the first level, I want to leave fortification on the north side, so that marksdwarves can fire through the fortification. Upon reading about fortifications, I figured I should build a second level. I want to place some Ballistas on the second level, in order to take out any major threats sieging my gate. Now, since fliers can fly over fortifications, and the Ballista would be on the second floor, should I place fortification in front and a staircase so dwarves can get up to load them? Or will fliers simply fly over the second wall of fortification and use the stairs to get in?
Most importantly, however, can a Ballista target an enemy on a level below, as in the ground in front of my fort? Or would they only be able to target above that (which would effectively make them useless. Also, when building a moat to protect my fort, should I use channels from the nearest water source? Or is there a better method for building moat.
As always, thanks in advance.