Very enjoyable game. I don't know why people are making out like it's impossible, I've cleared each level with varying strategies, it's just a matter of working out what works and what doesn't.
It seems the damage of the basic gun, machine gun and cannon towers doesn't increase as much as it should, and the poison tower also seems really weak.

I beat it with 4 Towers!
If you wanna easy win than do this.
Build a Machine Gun Tower into the mid than you need to upgrade range! of the Tower until it has nearly th whole map. Than upgrade speed befor build next tower . (speed to 8)

You skill first the money per kill skill.
Set Points in there until its full.

Than buy later a Freeze Tower, than schock and last Flametrower.

Skill your tree for: First Money, than ExP for kill, damge up for all towers, than freeze skill, than, epx at end, than schock and at least fire.

Your towers will gain high lvl and dmg, so not even one monster will pass that!

the waves get really hard to kill....really fast perhaps a quick patch would help things.

I can't even get past round 7. If it weren't so unbalanced, i would give it a 9. If you make a sequel, please make sure it's balanced.

this is definatly not a very good td game compered to the ones i have played.
it's just not that balanced cause the enemies can get a bit to strong for all your turrets to defeat, also it is sort of annoying how you can upgrade range very cheaply but reload is very expensive when you wanna focus on just building turrets and it is also annoying how you can't upgrade power alongside them.