To play it you only need a Source game from Valve. Half-Life 2, Counter-Strike: Source, Team Fortress Two, Portal, yadda yadda. The cheapest is Half-Life 2: DeathMatch for $5.

http://www.SteamPowered.com/
http://www.KingdomsCollide.com/
http://www.ModDB.com/Mods/Kingdoms-Collide

The game is medieval themed with magic, third person behind the player, action based with very slight rpg things(Only a few levels and you pick a new skill at each level).

The game has quickly become a favorite of mine, however it's oddly not very popular. I guess cause I keep slaughtering everyone and they leave ):