Elemental Weakness!

Some waves are weak to one damage type: enemies take +50% from towers of that type, and the wave panel shows which one is active.

The list grows as you unlock the towers that answer it. From wave 8 it is projectile — Gun, Cannon and Machine Gun — or flame . Explosive , the Missile, joins at wave 10, then frost at 18, bolt at 22, laser at 26 and poison at 37.