Nice game BUT!,
1.Even used wisely, upgrades and such still costs too much.
2.Nice touch with the equip items, but those couldve been part of the level up feature.
3.Try changing things around a bit, different types of shroom towers as opposed to one that you can equip items to. This makes it easier to defend in your own way, cannon types, shotgun types all that. This also includes those element stones as well.(instead of buying stones, make towers with the element already attached)
4. May I suggest cutting the prices of everything (yes, everything) in half, or maybe even more. Looking at how much damage I'm doing after a level up, it's just not cost effective to upgrade all my turrets, but I have to in order to defend myself.