Sky Citadel is the eighth area of Sonic Boom: Rise of Lyric. It is a colossal and marvelous city from the Ancients, located high up in the skies above Bygone Island, held suspended amongst the clouds through mysterious technology. Virtually untouched by time, it is an archaeological marvel.
In the quest for the final Chaos Crystal, Team Sonic journeyed to Sky Citadel to find the Sky Crystal. However, they were tricked by Lyric to relinquish their Crystals to him in exchange for Sonic's life. Not beaten though, Team Sonic pursued Lyric through the floating city to stop his plans to destroy the world.
A thousand years ago, the Ancients hid away the Sky Crystal, one of their own Chaos Crystals, away in Sky Citadel so that Lyric would not be able to find it.
After Team Sonic reactivated the tower leading to Sky Citadel, they walked towards the beam to carry them upward. Knuckles and Amy were the only ones to use the beam because in Tails' words, it was only designed to take two; thankfully, he had a plan. Meanwhile, Knuckles and Amy had begun traversing the floating city in search for the final Chaos Crystal. It proved to be difficult as the structure's strength is very weak and unstable. Halfway through their exploration, they see Tails flying his plane with Sonic on board with him, looking for a place to land.
Soon Knuckles and Amy find the Sky Crystal laser protected in a tower, and promptly shut the protection off just as Sonic and Tails arrive at the scene. The team gladly claims the Crystal, only for Destruction Troops to appear, surrounding them. Lyric descends into the tower, explaining that since Sonic had the map, all he had to do was let the team find the Crystals for him while he watched; now he wants the four to give the Crystals to him. Sonic refuses, but once he did, Lyric threatens Tails, Knuckles, and Amy that if they didn't, Sonic would die. Not wanting to risk their friend's life, they do as Lyric desired and dropped all the Crystals to the ground, much to Sonic's displeasure. Amy demands Lyric to let Sonic go as part of their deal, but Lyric reveals he had tricked them by ordering his robots to attack, shooting Sonic right on target.
As Lyric leaves with the Crystals, Tails, Knuckles, and Amy dig through the rubble, in fear that the worst has happened. To their relief, Sonic survived the onslaught, telling the others that they had a snake to catch. As they were leaving, they discover that the entire citadel is slowly collapsing from Lyric's robot army. Team Sonic fought their way through the enemies, the falling pillars, and the many traps, they reach the plane Tails conveniently parked far from their starting point.
As everyone was getting ready to leave, a chunk of stone broke the road, causing the plane to fall through, forcing Tails to start it up and fly. Sonic was left on the road, running after the plane as the road was collapsing beneath him. Eventually, he was able to get close enough to jump in the plane; he even landed in the window seat, much to Knuckles' annoyance. The team began flying towards Lyric's Lair to stop him once and for all.