Instructions
Play through the game until you have gathered 10 relics. Relics are given to you whenever you beat a time trial in a level. To start a time trial, walk into the glowing gold clock at the beginning of any level.
Beat the game and wait for the credits to finish rolling. After the credits finish, you are transported back to the warp room. Stand on the large gem in the middle of the warp room. This takes you to a secret warp area.
Stand on the third warp panel and then jump into the bubble to go to level 27, "Hang 'Em High." You completed this level earlier, but you are now taken to another part of the level that was previously inaccessible.
Make your way through the level by following the path and defeating enemies. At the end of the path, pick up the yellow gem. Exit the level by jumping through the bubble ahead of you.