Grey Alien
The mainstay of the emperor's army.
Flying Alien
Flying enemies are able to cruise over your defenses, and are immune to mortars.
Spawn Alien
Spawn enemies break into two grey aliens when defeated, and can sometimes can leap over junctions!
Regen Alien
These regenerating aliens heal over time, so you must kill them quickly. ElectroSlow towers slow their healing abilities.
Armored Alien
The armored alien is resistant against certain types of attacks, but weak against all others. The halo below the alien reveals its armor type. Light Armored (blue) aliens cannot be hurt as easily by energy weapons, Heavy Armored (red) aliens cannot be hurt as easily by explosive weapons, and Bionic Armored (green) aliens are resistant to lightning weapons.
Crittasaur
Crittasaurs are extremely fast. Look out for the Crittasaur King in later levels.
Bosses
Bosses come in many forms, and are led by the strongest boss of all, the Emperor himself.