[energy123]

Consider Legion TD 2 or Bloons TD 6 in Steam, they have some co-op. I recommend Bloons TD 6.

[Bombthecat]

Yeah, but those games are not free place, wc3 tower defense were like: here is your grid, go wild! The coop part was where the grids met or you pushed the monsters with a labyrinth from one side to another.