Thanks for playing - again - Colin! Level 3 was a last minute addition with almost no playtesting and the issues you've raised are spot on. Enemies should be able to break through stuff if no path can be found to the base; simply because players can otherwise just place a bunch of rocks/trees in a neat shape around the base and be done with it. We might also add some more restrictions (either via level design or new mechanics) on where stuff can be placed to prevent this sort of play.