"Mirrors"

There isn't much time for preparation, so I would suggest mixing random selection and drafting. For example, each player drafts 1 or 2 castles, and the remaining castles are chosen randomly (HvRulz could do this), so there's a chance to remove a castle that has changed significantly and for which they haven't had time to practice. A fair trade for heroes. There won't be immediate complaints about "Luna is overpowered" or something similar. If you want Luna, pay the price that reflects how much you believe in her.