Among the greatest
This is a great opener for a series of tower defence games based on this. The three towers are all awesome in themselves, but I was hoping for more. But at least you covered the basics, which is awesome. I enjoyed the skill tree that you set up, but perhaps you can set up a forth tree that belongs to you, the commander.. or gaurdian. whatever you are that set up passive and special abilties. Like first you start off with just the tree clearing spell, but you can unlock more along the way. And for the towers themselves you could set up different branches for whichever you want to go, like power, speed, range, skills and the such.
The different tiles are also awesome and a great change in pace. You could try to have more unique tiles as well, such as swamps, and graveyards that set up different upgrades or allow you set up one of two towers.. like the swamp can set up den's or crypts for a more flexible battle pattern.
The level designs are also great, but you should set up bonus features or side missions like "Defence of the mountain peak" which would be an all snow map, and would be a crypt only map.
I'm sure you won't make any changes, because this is only an output for a website I'm sure, but maybe the creator of this game or someone reading this might take this advice and put it to use in a game they are working on.
All in all this is a great game, and I am hoping for a sequel.