Boss Gate

The Boss Gate is an entrance/portal which appeared in Sonic Generations. As its name suggest, the Boss Gate leads to one of the game's bosses. They are located in the White Space.

Layout
The outer layout of the Boss Gates is a floating portal made of gears and locks. Just like the other places found in the White Space, the Boss Gate leads to a different point in Sonic's history, but instead of just a location, it brings Sonic into the middle of the battles with some of his biggest opponents (The Death Egg Robot, Perfect Chaos and the Egg Dragoon). In order to open a Boss Gate, the Sonics has to collect three Boss Gate Keys for each Boss Gate, which is scattered throughout the White Space. After clearing a Boss Gate, a Chaos Emerald is restored from its colorless state and passed on to the Sonics.

Unlike the other areas in the White Space, which leads directly to a point in Sonic's histroy, the Boss Gate's entrance first leads to the interior of the gate, which features a rather large black and white hallway, decorated with white chandeliers and (at least in the Death Egg Robot's Boss Gate's case) a wall with transparent glass cutting through the middle of the room which. Each end of the hallway has a stone door that leads respectively to the boss and back to the White Space.

Trivia

 * It may be that the Boss Gates was created by Dr. Eggman as one of the traps he had set up for Sonic; their layout suggest the places they head to is was not torn away from their place in time as the other areas, the gates is mainly constructed out of gears, which is associated with the Time Eater and finally it appeared that Eggman had expected Sonic would pass through the Boss Gate when he about to fight the Egg Dragoon.