I think Kingdoms is more a social political construct than is necessary. The Castle theme name implies construction and a common theme around the world of fortresses. IMHO, they should leave it be as the link to brand identity and the history of the theme will be a asset to the line.
I think if you look at the retail slowdown of the current Castle line as being their impetus for a name change, then what they really need to be asking is not the question of the name, but rather the product inside the box. Less of the machines of war and more of the life around the castle with some conflict elements thrown in for good measure of fun. Retail should have gotten the Medieval Market Village set, not the Ogre Fortress if they wanted to drive some sales along. Build up the main forts with some constructions to embellish it.
I'd go for just about any historical location and timeline withing the theme of Castle as it pertains to fortresses and fort towns.
There is no life I know to compare with pure imagination. Living there, you'll be free if you truly wish to be. -Willy Wonka, 1971-