nice concept... poor balancing
when you can win the main storyline by using nothing but fire towers (and trust me, I just did) when your supposed to have to use a combination of them... something is wrong. seriously wrong.
pretty much, it does the most damage hands down. and on top of that once you upgrade it only 1 time (on later levels you don't even need to wait for more income before being able to do that) it can reach anything in the game, even catapults.
if I might make a suggestion... make the fire tower a MUCH shorter range (and possible raise the fire rate of it some to compensate) this would at least force you to at least WANT the other towers
other then that, the graphics were fairly the same and flowed well with what you had, and I saw no real bugs so defenitely hats off to you for that.