Are the monsters actually disappearing into the walls, or do they spawn in the walls? The reason I ask is that a lot of the MM games have had the occasional monster that spawned in the wall - this problem goes 'way back. For example, there was either a demon or a devil that spawned in the wall of the Volcano Cave in MM4, and you occasionally get a dragon stuck in the ceiling of one of the rooms in Kriegspire Castle in MM6. But a monster appearing in the open and then disappearing into a wall would be new, TTBOMK.