Toxic Fortress is the final level in Crystal Caves. This is also the final fortress level in this game as there is no fortress in Bombard Cliffs. It is unlocked via completing D-3: Spine Springs and finishing that will unlock a shortcut to 6-3: Fog Cemetery. This level takes place in an underground fortress filled with poisoned water. Some of the floors/walls drop upon touch, so be careful. There is no boss in this level as with all of the lettered world fortresses.
- Activate the switch before it's obscured by the poisoned water to reach the coin.
- Before the checkpoint, go fast to reach the star coin before the wall obscures it.
- You'll need to activate the Yellow*, Green**, and Red*** Switches to access the warp pipe before the flagpole. Once inside, watch out for the spike balls and use the empty platform to reach it.
*Can be found in Yoshi's Island.
**Can be found in Sakura Village.
***Can be found in Crystal Caves.