What about those invaders that can destroy doors?
Create the wall as far as you are allowed and surround the zone, it will still have the same effect as forcing their spawn point - Just means some things might be delayed depending on where they spawn.
I love the second sugestion, and plan on working on that once the walls are done, but in the mean time I think I may try to get some extra temporary wood cutters out there long enough to get a massive amount of wood stored up, so I have less reason for them to be wandering around outside in the mean time.
Unfortunately, I kinda need the doors in the walls, so that it doesn't take too long to get to the trees, as I need the wood, both for fuel, and for cnostruction.
So far the plan for the walls is to have a double wall, with one tile of open space between them. The open space will be covered with wooden floors on the upper level to allow later building of fortifications along the sides. I've just dealt with two goblin ambushes. Fortunately,my only losses were a few dogs, as I was tipped off by both the dogs, and the dropping frame rate. At the same time I got some new migrants, including the dungeon master, and the elves showed up to trade. Both goblin squads were headed by dwarves, one died, the other got away, but all the goblins died. I'm working on some less-than-glorious tombs (read: holes in the walls where it's most convenient) for my corrupt fallen brethren.