This stage takes place deep within Dr. Eggman's pyramid base, filled with bonfires, Badniks, and a large machine in the center.
After locating the entrance to Dr. Eggman's pyramid base, Sonic and co. find that the door to a rocket that will take them to the Space Colony ARK, where Eggman is located, is locked. Sonic persuades Knuckles to find the three keys to the door so they may proceed.
This level, like some others in the game, has a large number of ghost-based enemies in it.
The primary goal of this level is slightly different from all the other levels belonging to Knuckles. Instead of finding Master Emeraldshards, he's actually looking for three keys. King Boom Boo is the following boss after this stage, and the Egg Golem is faced immediately after that.
Level Up Item
The Level Up Item in the stage is the Hammer Gloves, which allow Knuckles to break open iron boxes. The Hammer Gloves can be found in the hall from the first room behind a wall of crates.
Collect 100 Rings
After grabbing the six Rings in front of Knuckles at the beginning, the player should head for the blue section of the stage. They can play the Mystic Melody at the Ancient Ruin to make fourteen Rings appear. From there, they should keep traveling toward the blue section, where a Magnetic Shield will be in the room just after the metallic hallway. By arming Knuckles with this power-up, collecting the remainder of the Rings will be much easier.
Find the lost Chao
The player should play the Mystic Melody atop the Ancient Ruin on the floor of the Pyramid Core and dive into the water when the hatch opens. As they dive down, they should see a closed door. They must swim to the other side of the room and find the Hourglass, flip it, then quickly go through the door before it quickly closes. Once the player has made it through, they should swim across one hallway where two robots are firing, then another hallway lined with Boos. Once they enter the large cavern, they should swim up almost immediately to emerge from the water.
The player should climb out to see an Hourglass in the alcove above. This opens the door directly above it, and behind that door is the lost Chao. The player will not be able to climb straight up to the door, so they must hit the Hourglass and work their way upwards before the door closes.
Find the 3 Keys within 5:00
This stage can be a bit confusing, so the player should not be afraid to use a Hint Box for hints.
Clear Hard mode
To find the first key, the player should begin by digging through the painting found in the hallway between the red and green sections of the stage. They will then enter a large room with an Hourglass on the opposite side. The player then has to flip the Hourglass to make platforms slide out from the left and right-hand walls, and should then jump onto one of them and smash the Iron Containers to reveal another Hourglass. Next, they must flip that Hourglass, making the large hatch in the center of the room open. Now, the player should quickly run down and dig into the dirt there to reveal the first key.
For the second key, the player must head for the location of the lost Chao. Once they reach the large cavern, however, they must not swim up to the room above. Instead, they must swim straight ahead, where they will find the key behind an altar. However, they must beware of the Unidus hidden behind there as well.
After obtaining the second key, the player must swim up into the room where they found the lost Chao. A giant Weight should fall on the player and close off the entrance to the water. First, the player must climb straight up and hit a Switch to raise the weight again. After that, they have to flip the Hourglass in the alcove below and quickly dive back down. For a brief time, the key will be revealed in an alcove of the second step from the bottom.
Chao Container locations
In the blue area, the first Chao Container can be found in an alcove, which is located in the room accessible through the local Warp Wall.
In the green area, the second Chao Container can be found in the last room of said area.
In the Pyramid core area, the third Chao Container can be found on the lowest level, below the platform of the red area entrance.
In Death Chamber, Big is normally seen beyond the grating of E-01, from the red room's exit position. In the Hard Mode version of the stage, he has been moved slightly to the left from his previous position and downward.
This can possibly be explained by the fact that Rouge was breaking into the base while Eggman was gone, while Sonic, Knuckles, Tails and Amy are breaking in while he's still there (which can be deduced from the fact that Eggman shows up in person after the King Boom Boo boss fight). Thus, it makes sense that he might disable the Egg Beetles while he's inside his own base.