Okay, beat all levels. There is one very very very big flaw in this game.
When you build a wall or tower the pathfinding can go hay wire and send the mobs on a completely different route. This is especially crass on the level 3 mountain map.
Conversely you can easily break the game by placing and deleting a wall that triggers such a massive route change then and have A* literally walk the mobs in circles. Randomly having to wall off some island parts to exchange the length of alternate routes by 1 so the mobs were forced to run through my maze felt really weird.
I recommend playing some other tower defense games like Wc3 funmaps. Concepts like waypoints that mobs have to visit can improve on this.