The Palace of the Four Swords is a secret dungeon in the Link to the Past GBA version. If you have beaten the Four Swords multiplayer game, then this dungeon is unlocked. Inside of the dungeon you have to collect four swords to unlock a door to the boss. This dungeon's difficulty is from normal to hard. Once you collect the four swords, you unlock the boss room and then you have to face four Links that correspond with the Four Sword color Links. Once you have defeated the purple Link, the credits roll and all that jazz. I found this dungeon to be pretty fun and it is located at the Pyramid in the Dark World. I would recommend you have all upgrades and have beaten the game before entering this place. Also bring some fairies and red potions. The Hurricane Spin would be somewhat helpful. The Links are pretty tough, so watch out!