The game features different multiplayer modes for up to 8 players.
Duo Campaign: You can play the campaign with a friend, with one player placing towers and the other player controlling support abilities (2P)
Co-Op Campaign: Each player builds their own defenses and uses their own abilities; more players = harder difficulty (2-4P)
Cooperative: One-off map played vs AI (2-6P)
Co-Op Survival: One-off survival mode (2-4P)
Wars: Tower Wars mode from any other TD game -- you send creeps that attack the other players, while also building your defenses (2-6P)
Versus: Two-team last man standing co-op mode. You don't send creeps, you only compete with the other team to outlast them (2-8P)

All modes can be played via Internet (Steam lobby) or LAN.