Since ancient times we have pondered this question: Why did the blob cross? Well alright, in question is actually not the blob, granted, but let's just set this aside.
For some unknown reason these blobs are trying to cross. Maybe they just dislike the dirt covered ground? Maybe be not. But instead of actually wondering about this question, why don't we just get to the root of the problem?
THEY. SHALL. NOT. CROSS.
Defeat all incoming blob waves using your towers. Carefully construct your maze, but be aware: Mobs will be coming from west, north, east and south trying to cross.
The game is aimed for replayabiliy and fast-paced.