rewards aren't worth the frustration.
it's nice that it's difficult, but the cash income doesn't scale well with the difficulty. like those armor bugs on the 5th wave in stage 1, for example; the shotguns are too slow, and the machine guns are too weak. there's not enough cash to place enough machineguns or shotguns to kill them before at least one leaks through, ruining a perfect run so far.
the slow towers seem to be useless, and just a waste of money.
tower range seems to be lacking
upgrades give you severely diminished returns; it's better to buy another tower than upgrade.
all in all, there's potential, but this game feels like a domineering girlfriend; you get the rewards, but only if you follow her exacting standards that are never clearly spelled out and seem to be constantly shifting, and even then, only if she likes you that day.
games need a certain amount of freedom to them; forcing every player to do the exact same thing or lose, isn't difficulty; it's bad design.