I really enjoyed this game. Getting all the medals was a challenge but not impossible. I find nothing wrong with the game except a few minor spelling errors. I would suggest fixing them if there is going to be a sequel.
I notice a lot of people saying that this game is too hard. Here are some tips for making the game easier-
Plant towers where they will cover the most ground.
More towers is better than just a few highly upgraded ones.
Use acid (Poison) towers at the start of where the bad guys spawn this wears them down. Space them out along the road so they get worn down by them so your other towers will be more effective.
Use slower towers. While the bad guys are slowed down your towers will hit them more times, making them easier to kill. Put them next to the acid towers.
Make sure to use cannon towers and speed gun towers in addition to these two towers.
Plant bombs near the end. If you put 3 or 4 bombs near the end these can be a safety in case that one monster gets away.