....
This brings about a question though ... how are ya'll defending your trade depots with a 3 tile wide opening to allow for wagons? Is it feasible to simply destroy and rebuild the wall every time a wagon caravan shows up?
My trade depot is always behind a tunnel that is near my main stairs.
Its defenses depend on the location. Normally I try to put the main entracne (that leads to the trade depot) at a height of 1-2 levels above ground, reachable only by a retractable bride (that can be retracted by a trigger that is within good reach of my dining hall (= meeting hall).
In my current fortress however I was forced to make the main entrance at ground level (just a couple of ramps leading one level deeper to the tunnel that lead to the trade depot.
Therefore as soon as I could I built a keep atop the entrance, as well as 2 rings of castle walls around it. You can only get past these 3 lines of defense (each independent and able to fire at the next outer line of defense, should this be breached) by a long way of platforms and retractable bridges that are 1 level above ground. (with a guard animal being chained at the place where ramps lead up to the first platform, so that kidnappers, thieves and ambushes can be detected before they reach the first wall.
All bridges are (in groups) connected to levers next to the dining hall.
Well, at the moment, as my miners had some time at hand, I also made a long tunnel (protected by bridges with moats, as well as several cage traps) that leads to an entrance near one of the spots near the map border, where usually the human and elven caravans tend to appear, so that the wagons are better protected from ambushes and thieves that might be active in the countryside (a human caravan wagon detected a kobold thief once; obviously the Arnold Schwarzenegger among the kobolds, as he was able to demolish the wagon before he fled, causing the caravan to iummediately travel back home)