It was fine at first until the mobs started having red blue blue orange red purple defs at that point I just stopped. Way too random as most ppl have suggested.
It seems like after playing awhile x2/x4 speed slows down the game especially the attack rate hence it may seem harder. Refresh the game and you'll see that things seem to move alot faster (attack speed as well). And archer's shouldn't have the blind spot for air units (They can't look up?)
The game is riddled with glitches poor AI and quite difficult. Lack so many simple things for a TD game such as a "start wave" button. Like previous comments stated had potential but ultimately fails..alot