Zen Studios recently released an update for their tower defense brawlfest, CastleStorm - Free to Siege. With the new PVP mode you'll be fighting other players from all over the world. Build up your castle, design your defenses, and siege away against your enemies. You can play as one of four different nations and dominate the map for weekly rewards.
Bring down the castle by downloading CastleStorm - Free to Siege for free (obviously).