Average
Was fun for a bit, got all the upgrades at level 13 so there was really no point of getting more money. The upgrades were disappointing, it would have been nice if you could do stuff like shoot faster, have exploding projectiles, or multiple shots from a tower.
Honestly, I just got bored and didn't want to play the last five levels.
And just a hint, ads after every level is NEVER a good idea...