×beyond dropping all the trapdoors. I'd need some feasible way to trap the workers, I suppose.

×And there's not much in the room you can use anyway...

×wait around until the mud fills up the room. But that still doesn't yield a workable configuration where I can remove those trap doors. Because there'll be a cluster of workers at some point near the last corner the tar reaches, and I can't cross that or they'll repair all the traps.

×When you have dropped a trapdoor, and there's mud outside of it the builders for some reason prefer to target other trapdoors.

×

When you have dropped a trapdoor, and there's mud outside of it the builders for some reason prefer to target other trapdoors.

I think that's the path-finding algorithm at work. I believe that after resupplying, workers will head towards the closest build marker. If mud blocks off a square from the outside, then the only path will be from the inside of the bridge. And such a path will almost always pass another build marker first. I didn't figure this out when playing the room, and in fact wondered why the workers never built on such squares, but I think that must be it.