I came up with the idea when thinking about how to reverse roles in some of my favorite genres, including TD games. The first idea was being responsible for spawning enemies instead of building towers, but that's already a game mode is Bloons TD 5/6. Then I realized you could make a pretty good puzzle game by manipulating paths, destroying towers, etc. and made this game.
Glad you enjoyed the game, and the suggestions for additional mechanics are really good. Also, there's a playthrough video now if you want to check the solution for level 4.