When Eggman's detector senses two Chaos Emeralds belonging to Sonic and his friends (including Tails' fake one), Eggman climbs in his Egg Walker and sets out to the ARK to confront Sonic. He captures Amy and blackmails Sonic into giving him the Emerald in exchange for Amy's life. He encapsulates Sonic who attempts to give him the fake Emerald and sets the capsule to explode in space. He then bargains with Tails to give him the real Emerald in exchange for their lives. Tails, believing Sonic has died, is angered and instead fights Eggman to avenge Sonic.
Collect 100 Rings
There is an abundance of Rings in this stage. As such, if the player grabs each one as they go, collecting 100 Rings in under a minute and a half should not be too difficult.
Find Lost Chao
First, hover to the catwalk above the second Point Marker, and destroy the pyramid atop the platform on the right to reveal the Ancient Ruin. Play the Mystic Melody near the shrine, and a Spring will appear. This Spring can bounce the player to the first of many platforms that must be navigated. Just keep going up in the direction of the arrow signs. Eventually, the player will reach a ledge with another platform. Here, the lost Chao can be found inside the pyramid.
Reach the Goal within 8:00
By hovering, the player can bypass many intermediate platforms, but be sure to free-fall straight down shafts rather than hover and risk messing with enemies. There are also opportunities for more than enough points while the player is riding the rails.
Clear Hard Mode
There are a lot more enemies in this mission, and they require quite a bit proficiency to deal with. The cylindrical mechanism that chases the transport Eggman uses at the end of the stage is significantly faster.
Big the Cat sighting
When boarding the final rail ride, the first plaform the player passes under will be one with laser turrets on it. At this point, Big can be spotted directly above the player, hiding behind a laser turret.
Chao Container locations
After the second Point Marker, the first Chao Container can be found inside the pyramid to on right of the next platform, close to the Ancient Ruin.
The second Chao Container can be found nearby the fourth Point Marker.
In the room that comes after the seventh Point Marker, the third Chao Container can be found in the back of said room.
The Stage takes place in the outer portion of the Space Colony ARK, and as such there is low gravity making player jump higher than usual, and even enabling flight for a short amount of time through hovering. There are portions of the stage where the Egg Walker is placed on a transport system that restricts its movements. Near the end of the stage Eggman finds a cylindrical mechanism closing in on the transport he's on that he must shoot at to slow the progress of. The large amount of GUN robots and passive objects make point collection easy, making the Stage one of the easiest stages to earn an A Rank in the Dreamcast version. Later ports of the game increase the score requirements to get A-Ranks here.
Level Up Item
The Protective Armor is found in this stage. It is found under a platform after the second drop location. One must use the rocket (across from the Level Up Item) to break the cages to allow access to item.
This is the only level that a player can score 100,000 points or more
Cosmic Wall is notable for having the highest score requirements for A-Rank of any stage on Non-Dreamcast versions of the game with Hard Mode requiring at least 100,000 points to get A-Rank.
This is the only stage in the game where gravity is heavily reduced, as well as the only shooting stage to feature a chase sequence.
Aside from including the scene where Eggman holds Amy Rose hostage and forces her to divulge Sonic and Tails' location, the scene that led into the boss fight with Tails cut out the scene where Sonic considers using the Fake Chaos Emerald to escape from his predicament via Chaos Control, presumably to avoid giving away that Sonic survived.
In the Xbox Live Arcade and PSN re-releases, grabbing the Chao key is dangerous as it is very common to be sent to a Test Level instead of the Chao Garden with the only way to escape is pausing and exiting the game.