This game has a pretty solid concept and system behind it. The wizards' dependency on one another is well thought-out, and the implementation of different features and capacity for more units as you level up makes sense. I do like the art and music as well, it works.
My biggest critique is that the wizards' AI isn't as intelligent as I had expected, and as a result I had to have a good number of certain types of mages (most importantly fire mages and druids) to keep me from having to either wait a long time for trees to grow or scramble to keep snails at bay. The closest thing I had to a flawless system was at the 15 units level, where I had 5 druids, 2 water mages, 6 fire mages, and 2 necromancers, and even then I had fire mages who would not recognize snails right in front of their faces and other mages who would just walk right in front of them and into the snail's path.
I also wish that there was a little bit more GUI, in terms of being able to see how much progress is being made through the game (xp, levels, crystal charge) and having the buttons at the bottom right (other than escape) explained. I was initially confused when I heard the charge-up sound but my units were full, considering it had sounded like I'd unlocked something but there was nothing for me to do before the level where snails were introduced.
Finally, I've got a couple tiny nitpicks. I think it'd be nice if you could tell apart the mages' powers by sound, since that would be a way to tell what's happening without having to glance around the screen. And secondly, it'd be nice if the snails were destroyed when the ending animation was triggered, rather than still generating. It made me feel bummed out to see my wizards helplessly dying to the snails while I was leaving.
Even with all that, though, I still loved the game and I still want to see where it goes from here. It does need a little bit of work to reach it's full potential, but overall it's really solid. This was a fun way to wind down after a long day - thank you for making this game!