[foopod]

I have been playing a lot of Mindustry lately, not just free, the game is open source too.

"Mindustry is a hybrid tower-defense sandbox factory game. Create elaborate supply chains of conveyor belts to feed ammo into your turrets, produce materials to use for building, and defend your structures from waves of enemies. Features include a map editor, 24 built-in maps, cross-platform multiplayer and large-scale PvP unit battles."

https://anuke.itch.io/mindustry