Pure tower defense: progress by killing biters, dense nest placement, periodic waves with bosses, Master Ammo Chest system. No pollution attacks, only evolution.
The rewrite version of this mod has been uploaded! https://mods.factorio.com/mod/tower-defense-evolution-rewrite
please if you like this mod download the new version of it, as this mod will not receive any more updates as the rewrite version will take over.