So, it's either your favorite, or the one you'd rather not play.
I don't think it would be fair if, for example, a strong player playing as Rampart, ended up with only Ramparts in their group, then in the playoffs, and so on throughout the tournament, and this is actually a slightly exaggerated version of your suggestion.
But my idea is different. If you're playing as Rampart, does it matter who your opponent is playing as? (considering that they can choose any castle, based on the rules)
I'm not even talking about the fact that it's more advantageous to play as Tower against Tower than Tower against Dungeon.