Just started playing again, I didn't realize you could stack enemies on top of each other to level them up. Will have to explore the ramifications of this. You may want to mention that you can do this.
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.