I enjoyed it
I have a few critiques, on gameplay structure though. The leveling system is extremely flawed. While most tower defense games go for a "upgrade for a certain amount" system, this game went for a "for every hit/time this tower is used, it gains XP" system.
Now the flaw of this system is that different towers are on the same system, so while the Dragon may level up in a few waves, it literally takes four or five times the amount of waves to level up a Hive. And the only reason why is that a Hive is a single hit tower, and the Dragon can hit multiple enemies at once, making it a much faster XP gaining tower.
Another example of this flaw is the Mudman tower. The Mudman starts as a single hitting tower, and grows at level 2 into a mass lane attack tower. The progression to level 2 is a long one, and can take several waves to achieve. However after level 2, the leveling becomes much quicker, pretty much the opposite of what you want to do.
A system of XP currency would have probably been better, so you can choose the towers in the back of the line to be upgraded in case something bad happens, or for hikers.
However, after I realized that the combo of slime + dragon + mudman + mudman + plant on a higher lane, with hives filling out the rest of the lanes and nothing but level 1 skeletons on the first level (to take the brunt of the warriors and bombers) was basically a win button, I beat the entire game on extreme mode.
Overall I would love to see a way to customize leveling, including leveling spells and a variety of spells to pick to take into battle.
Very much hoping for a sequel.