Agree with the laser tower part. Also the manual mode feels WAY better then the towers shooting for themselves. I cleared the whole game (except the last level) perfect with only Laser Tower fully upgraded and Magnet tower upgraded.

It kinda feels like the normal towers are dumb when it comes to shooting. What also might be cool is to add resistance. So you don't have to make new creeps.

Example: You can choose to upgrade your towers to bullet penetration or explosive damage. Then you can have creeps that are resistend to these effects so it will be harder.