The Molten Fortress (溶岩城, Lava Castle) is a sub-world found in the Light Realm within Adventure Mode in Super Smash Bros. Ultimate. It is located in the north central region above the Cliffside Rapids, and unlike other sub-worlds in the Light Realm, it does not require a certain type of Spirit to reach it. It is themed after the castle stages within the Super Mario series, and as such, P-Switches and Pipes are required for use in order to progress to the boss, Giga Bowser. The area's background music is Castle / Boss Fortress - Super Mario World / SMB 3.
The Spirits found here are generally characters found in castles, or ones associated with fire and lava. Spirits are ordered by when they are encountered on the map, up until the boss.
The boss of this sub-world is Giga Bowser. Defeating him grants players the Giga Bowser spirit, the partial destruction of Galeem's shield depending on the other sub-worlds defeated, and Bowser as a playable character.
Two chests can be found in this sub-world.