Dan Zuccarelli reviews Kingdom Under Fire: Circle of Doom for Siliconera.
"In getting ready to play Kingdom Under Fire: Circle of Doom I must admit that I've never played any of the Kingdom Under Fire games. Hell, I didn't even realize there were any others. So I came into this game about as fresh as they come. No expectations, no previous experience. I honestly didn't even know what type of game it was."
In conclusion, he would not recommend the game.
"In talking to the people I played with (in online co-op), almost everyone was a fan of the previous games and all of those felt this was a step backward for the series as a whole. Common complaints included the disjointed storyline and the awkward leveling system...
While I enjoyed the time I spent with it while in multiplayer, I don't think I could ever play through the game more than once. It will probably satisfy series fans, and maybe even fans of the action-rpg genre. But for everyone else, you'll most likely want to look elsewhere. The combat here is far to repetitive to entertain most everyone else."