Onox's Castle is the final dungeon in Oracle of Seasons. It is the seat of power of Onox, General of Darkness, and where he has imprisoned the Oracle of Seasons in his plan to bring destruction to Holodrum.
The castle is located in the mountains of northern Holodrum, west of the remains of the Temple of Seasons and east of Tarm Ruins. Link can attempt to visit the castle early in the game; however, it is surrounded by a powerful dark energy that can only be bypassed using the Essences of Nature and will send Link back a screen if he does not possess them. The grounds of the castle are patrolled by Lynels who will further attempt to stop Link from advancing inside.
Inside, the castle is composed of a row of chambers containing various enemies — the first one an assortment of Wizzrobes, the second Darknuts and Floormasters, and the third a rematch against the miniboss Facade. Each room features side chambers that contain healing fairies, should Link need them, but upon exiting these chambers, he will be sent back to the castle's entrance hall and will have to fight his way through again. Following the fight with Facade is a vacant chamber containing jars with recovery items for Link to collect, as well as an open path to the room where Onox and an imprisoned Din are waiting.
The castle remains occupied by enemies even during the epilogue where Onox has been defeated.
- At two points in the game, Onox and a captive Din are shown in a room inside his castle that is never explored by Link. It has a purple color scheme and is lit by a set of four torches on the floor. This room is first seen during the game's prologue when Din is captured and again after obtaining the Huge Maku Seed.