Let me preface this by noting that I'm pretty bad at TD games, since most of my experience comes from the Bloons series.
I think your gameplay is really solid. The towers you do have are all distinct and have their own niche uses. The enemies as well have good variety, which forces me to rethink my defense layout. My only issue is that not enough money is rewarded from killing enemies. I think upping the amount per enemy by just one or two gold should be enough, since at the end of every round I find myself only able to maybe place one new tower down. I hope to see more from you mate, good stuff.