I agree with all the bug comments especially uberpenguin. The two most annoying bugs I've found so far: When the zombies break the first barrier, they go straight to the core, bypassing everything else. Also, when clicking on a Light Artillery Tower, the game freezes and you cannot upgrade the tower at all. You have to select a different tower to unfreeze the game.
I keep experiencing a bug where a wave won't finish. I can call the next wave and even beat the boss on level 3 but I couldn't finish the level because the wave before was stuck. It's happened to me 3 or 4 times now.
I agree with the inventory system. Either give more slots or make equipped equipment disappear from the inventory. But I wanted to point out something in regards to buying healing supplies. Buying the healing herbs that only heal you 20 HP is the best bargain. The amount of money you spend for the healing pots that heal you more increases at a higher rate than the amount of healing you get. For example, if you wanted to heal 1,000 HP, you'd buy 50 herbs for 750 G. You'd only have to buy 17 small pots but it will cost you 1,275 G. You'd need only 7 Large pots but you'll spend 1,750. You may be paying for convenience but if you want to save money, just buy the herbs.