In Creeper world the enemies you face are all different forms of the same entity the Creeper. It's the ultimate enemy as it cannot be killed only pushed back long enough to run away.
Emitters emit creeper in fixed intervals placing a fixed amount of creeper in the middle cell of their image. Then the creeper spreads across terrain. Because of this mechanic it's possible to cap an emitter by placing one or sometimes more blasters next to it, allowing them to destroy all the creeper before it spreads. It's possible to stack a few emitters on top of each other, with different strength and interval allowing some interesting effects. Emitters cannot be destroyed in Creeper World 1.