Onox's Castle is the final dungeon in The Legend of Zelda: Oracle of Seasons. Located atop the Northern Peak, it is the home and stronghold of General Onox, the antagonist of the game. To gain access to the palace, Link must have obtained the eight Essences of Nature and the Maku Seed. Unlike previous dungeons of the game, Onox's Castle is relatively short and linear. It contains no Small Keys, Treasure Chests, Dungeon Map, Compass or Boss Key. It consists of seven rooms, three of which contain no enemies.

In the final room, Link witnesses Din being encased in a crystal. The battle between Link and General Onox then commences. After Link defeats General Onox, the floor begins to disappear and Link falls to a new arena, presumably below the castle. Here, Onox morphs into Dark Dragon, and the final battle of the game begins.