Happy Wars launched in October 2012 and has seen more than 6 million players on Xbox 360 to date. The game supports up to 30 players (15 vs. 15), who must attack their opponents' castle while simultaneously defending their own.
On Xbox 360, Happy Wars requires an Xbox Live Gold ($60/year) membership, so it's not exactly free. The game also offers various in-game microtransactions, but these are of course optional. Happy Wars is developed Japanese studio Toylogic.