They make pools that change the scale of enemies, affecting their health and speed. You can even shrink enemies down to nothing! (they also do really cool things to the music)
You earn the resource to scale towers (called Bigcraft) based on the enemy health, so bigger enemies reward you more Bigcraft, so you're incentivised to make the enemies more difficult so you can scale your offense faster, but not so unmanageable that you overwhelm yourself.