It seems to me it's only natural to have one. There are plenty of available enemies:
Facehuggers for basic enemies, drones, queens that lay eggs, chestbursters that turn into drones, predaliens, even evil humans and androids. For your towers you could have:
Husdon with a short range but rapid-fire pulse rifle Apone with a flamethrower Vasquez with that heavy machine gun Newt, who slows the Xenos down by distracting them Hicks with a shotgun
And two elite towers: Riplet with a pulse rifle and flamethrower that can target different enemies at once, with a random chance to fire a grenade Wolf Predator, who fires a plasmacaster and attacks with melee weapons if they pass close to him, and has an upgrade option to deploy those laser net traps.
