It's challenging here and there, but it's gets to the point where it's literally just unfair to the player. One boss a number of levels in, simply deletes 80% of your towers. If you didn't purchase a boosts/extra items before going into the level, you just wasted 30 minutes of your life. All because of a cheap boss fight that requires you to purchase something to be able to beat him.. I've played others by this dev before.. they just get more convoluted with each iteration.