Thank you so much for the awesome comments! I agree with you, I also want more depth :). The different enemy types are "cute" right now, but there's not much reason to choose a particular one over apart from some very light differences.
The difficulty will be implementing it in such a way that it doesn't get overwhelming for the player. But indeed, more support towers would be nice. I already have trampolines on my list, much like the teleporters you wrote :D
Thank you for all the other ideas as well! I'll include them into consideration.
Btw regarding the enemy stacking mechanic: It doesn't add too much right now because the numbers are not particularly balanced and the mechanic is almost impossible to discover without a tutorial, buuut there is a strat vs. the Swarm Of Eyes where you can stack the Druid Mushroom in front of it to get massive heals for the peasant.