The map you created is really great. A lot of TDs I play have a few very obvious places to put towers (around U turns, near two paths on opposites ends, etc) but your map is absolutely loaded with prime real estate. I think some of the enemies are unfair. The long snake enemy on round 3 (I think), and the super fast one later on. Weaker versions of these enemies that introduce players to their concepts would allow the player time to adapt. Alternatively, the first time you see these enemies they could deal very little damage. You lose essentially all your health pretty early on.
I'm giving a lot of criticisms here because I actually really enjoyed this entry, I think you should give making TDs another shot in the future!