I have an interesting fort right now, "Ropedanced". Through a lucky placement and some careful digging, I have just one entrance to the fort, access to magma, I'm working on access to running water through a long-term engineering project and -- most importantly -- there is only one map edge, the northwest corner, from where caravans and enemies can reach me.
This allowed me to build a large-ish wall around a wide swath of land, giving me somewhat protected area for outside activities. I built a channel with a bridge and a sort of 'baffle' area to put the trade depot, and then a trap labyrinth behind that. All of this is outside of the fortress proper. There's some risk of course that flying creatures could bypass all of this, but I'm willing to treat that as a separate problem.
Here's the issue: I don't have a strong enough military yet to mount regular patrols to the map edge without losing dwarves. For two seasons running now, the goblins have assaulted incoming caravans. They completely slaughtered the elves, took out the human Merchant Baron, and then were killed by the caravan guards.
My question is: is there any reasonable way (sort of sprinkling traps around the map, which I am doing) to give the caravan some cover without actually exposing my military to unreasonable risk? Or are they simply going to have to fend for themselves?