Some towers are too expensive, you can basically get through the entire game using only Fast Towers, Basic Towers, and sometimes Super Towers. To beat some levels, you HAVE to grind on other levels, and while it may seem fun at the beginning to do that and get a head start on the next level, it loses its taste after a few times. Also, only the first 4 upgrades are needed, and depending on how much you grinded, bomb upgrades too.
3/5, not a great game, but still good.