×Why are there sets of two trapdoors leading between some of the sections?

×Hint - what can't roaches walk through?

×Roaches can't go through pits either. Stepping on trapdoors makes pits form, so you could use this to block off unwanted roaches from the left.

×Try collapsing the trapdoors to the left of the orb to block off attacks from the left, although make sure you leave one trapdoor behind there so the mimic can get to the orb later.

×You are correct - roaches cannot go through walls, so use the walls for some protection to your right. From a position like the one in the picture, you can use the pits to the north for protection. When a roach approaches from the left, move so the pits are just north of you so the roaches to the north cannot get you. Once killed, just resume your roach killing by moving right to be under the trapdoor.

However, you can try a slightly different method, which I think is easier. Start the room facing east (Leave it, turn to face east, and re-enter) Now in as few moves as you can, make your way to the first pair of trapdoors (Efficiency requires moving in diagonals round corners as well as straight lines). Ignoring the other roaches coming from elsewhere, the queen behind the force arrows on the eastern side of the room (The one you are going towards) will have spawned, and the offspring will be moving towards you. Kill those quickly, and you should then have enough time to kill the ones to the north before any more roached are created (Remember to move forward to kill if the line of roaches is only one monster wide). I think you should then be able to enter the orb chamber without too much trouble.