To be honest, I don't remember The Last Castle that well. And in trying to access it in our backend... I'm not able to play it to refresh my memory.
With that said, I can't think of another example of an online, co-op multiplayer, defense game that isn't a very large, standalone type of product. I went through our "Multiplayer" tag, but it's almost entirely MMOs or PvP multiplayer.
You might want to look into these games:
Dungeon Defenders
Sanctum 2
Tower Wars
Orcs Must Die 2 [Unchained! recently went into open beta]
Sins of a Solar Empire
Farm Fortress
Hope that helps you find something to fill the void, even if you have to go outside of Armor for it (and pay some cash dollars).