Almost impossible to lose
Tower defense games require careful testing to create a good sense of balance, strategy and variety.
This game had none of that.
I never once got close to losing and really never had much of a strategy... I just picked all fire or all magic upgrades and builds. The strategy of investing in more than one tower type for combined powers is a staple of tower defense games but this game actually makes building more than one tower type a bad idea since your build costs go up incrementally each time you build, while your upgrade dollars will end up spread between two different types of towers.
Aside from the above, each level really felt like the same as the last except for some extra units that never really were any tougher since I always had stronger towers.