Bolvangar

Bolvangar is a gigantic underworld headquarters surrounded by strong currents projecting far into the deep of the sea. It is an impressive sight in most headquarters of the sea, even more so for Atlantic Ocean. The headquarters itself is located in New York (underwater beneath New York) and served as Xemnas's another secret lair.
Containing two hundred and fifty-one floors and chambers, Bolvangar seems to be built both as a fortress and even a high-class hotel only for Xemnas and the Royal Adults. The entrance to the Bolvangar itself is obscured by a series of half-buried tunnels and dead-ends in the surrounding deep sea and has lot of invisible dangerous objects and the headquarters is hard to reach and has the underwater dome has a actual gigantic octopus in both underwater windows, unfortunately Bolvangar is also served as a prison and headquarters. 
When four keys are inserted into the base of the tower, the Bolvangar's hotel area opens. Impeccably decorated, Bolvangar is filled with exquisite paintings (including The Birth of Venus, Venus of Urbino, Venus and Organist, Ophelia, The Land of Cockaigne, Mona Lisa Smile, Maja, Storm on the Sea of Galilee, Rokeby Venus (NOTE: Julie "Venus" taking over Venus's place), The Birth of Venus, a Matisse painting, Girl with a Pearl Earring, A Sunday Afternoon on the Island of La Grande Jatte, The Physical Impossibility of Death in the Mind of Someone Living, Last Supper, Dogs Playing Poker and Hunters in the Snow), and statues of various Renaissance (including the statues of David and Venus, even statues of Xemnas and Julie "Venus"). There are several areas for guests - an information desk and reception area, several healing devices and PCs, a bar, a casino, a relaxing bath, a horse racing, a dining room and many other luxuries honeycomb the tower. At the very top of the tower on Room 251 is a giant, caféteria where Roger and his daemon are found.
In Room 250 (the tenth room of Bolvangar), it has a the big screen 3D TV, deluxe karaoke machine, PS3, PS4, Xbox One, Wii U and THX quality sound, as Xemnas said "that would make George Lucas and Jean Baptiste-Grenouille from Perfume: The Story of a Murderer cream in their pants" for Julie "Venus" as he revealed is a present for her.
It has a massive dome in the crater of Bolvangar, where Sonic and his gang battles Xemnas, his lover Julie "Venus" and his whole henchmen. There is also a labyrinth of lava rooms with unexploded bombs and lava sprites also as traps and strangers and Heartless Minion that Sonic and his gang must face in order to clear access to the direct elevator, Lascia ch'io pianga from Handel's Rinaldo (1711) is heard in the elevator, which Xemnas rides up in and it also has a machine express and a thirty-one giant hamster balls-like bubbles and some traps to watch out and then turn left. 
In the semi-final level, You'll be faced with a boss battle against the Snowball Guardian. There are three stages to the fight. Start by firing the charge glove at one of the generator lenses here. Wait for the guardian to finish discharging the energy to the remaining lens. Charge up one activator switch on each side of the room, making sure the colors match and throw the snowballs to the generator lens that gets powered up. It helps to be targeting one while the snowman is doing the melt Repeat for the last lens. While you're doing this you'll need to avoid the guardian's snowballs, just keep moving and you should be fire. The only differences in the second phase is that the lens are not charged to begin with and the robot snowball missiles in addition to the throwing. Charge up the first lens like the last two in the previous stage. Stay out of the circles on the ground to avoid the snowball missile. For the final stage you'll need to disassemble the covers from the three lenses first. After that it is just like the second stage. The fight is complete after throwing the last lens.
It also a garbage bin used as a gravity in the final level beneath the floor of Bolvangar used as a Summit.
During the battle, Sonic and his gang scale the debris floating through the rift in Bolvangar.