Beat the game and you will be asked to save your cleared data. Do so and the Secret Gate will be unlocked in the Main Menu. In it, you play Blackjack against the Princesses.
After beating a game once save the game and start from the begining. Then in the Florin kingdom go through Ressurection path (etrance - stone in north-east corner) and get Anne's Braslet. In later events, when Loue open rabbit hole, you fall in the creepe Maze forest. Go up (there are 1 fight), then in the cleared forest up, right, right, up, up. Talk with Mad Hatter and enjoy new good ending.