I have a related question about this too. In our first game the invader had about ten goblins behind cover, I have very little idea what all of his things do, so I moved a load of marksmen to fire on them. In the assault phase all mine miss and the goblins kill all my dudes on the walls. Is that right? Because it seemed quite strong..
First off, there should have been no more than 7 at a Rampart. But assuming you are remembering wrong.
Yeah, that is how it works. But because of the cover there, I wouldn't put your marksmen at that location. You can't shoot him but he can shoot you.
But at some point he is going to have to venture out from behind cover and assault the wall - which will shut down the ranged attacks at this location. So while it is a fortress for him, it gains him nothing.