I actually agree that a "natural" themed tileset ought to have levels using more slopes. A more "man-made" themed one should probably have a flatter, more streamlined design which has a logical element to it. A castle level should feel like a castle as a whole, and not just look like one because you used the tileset. But then again, I'm a strong believer in making levels that follow a sort of theme like that. This only applies to single player though.