If you are playing with expansions and using 8+ Princess Cards in the game at once, you can use this optional rule.

When backing a princess, the player can get one additional princess as a guardian, placing the card face-down in the Domain.

The abilities of a princess card placed in a player’s Domain as a guardian are not available, and the player cannot place counters on the card. However, since it moves the princess card from the center of the table, it is no longer available for other players to back.