When Eggman blows into the HQ on Prison Island, the security systems try to block him off by closing all the metal security doors, which must be destroyed in order to pass through. The goal is to destroy the G.U.N. robots and penetrate the entire base. Throughout the level, the security system will alert enemies to Eggman's presence by announcing the level he is on. The security of the level gets tougher depending on the security level Dr. Eggman is on.
The layout of Iron Gate is basically the same process with new twists when a new room is entered. Most of the time, the player must try to shoot locks off doors or destroy enemies while on moving platforms. Iron Gate gives the player an impression of what future levels are like.
As long as the player does not get hit, collecting a hundred Rings in this stage is fairly easy. To so in under 1:35, however, it requires that take a detour when the player gets to the room where the first level-three doors. Upon entering, walk straight ahead, and the player should see a large pipe with a spring at the end. Use it to bounce up onto a platform to the left, then destroy the Iron Container to reveal another spring. Bounce off this Spring and hover to a platform on the right. Here the player finds ten Rings and three Springs bounce up and land on the pipe above, where there is a row of rings the player can hover through before dropping back down and continuing on.
Find the Lost Chao
Here, Mystic Melody, large Cannon and Jet Engine Level Up Items are required. To find the Chao, the same detour that is described in the "Collect 100 Rings" must be reached, but drop down next to the Ancient Shrine instead of back onto the the main path. Play the Mystic Melody near the shrine, and a series of platforms appear, leading to a pulley. Ride it into the room above and head around the corner to the right. The player comes to a dead end where another ancient shrine is. Play the Mystic Melody again, and a spring and some more platforms appear. Work the way up, and find the Chao on the top level against the center of the wall.
Reach the goal within 4:00
This mission is very easy. Just keep firing and try to blast the doors open as the player approaches them.
Clear the Hard mode
The only significant differences here are the addition of more aggressive enemies and the disappearance of quite a few Rings.
Big the Cat sighting
In the first elevator shaft, stand on the bridge before the "Level 2" door. From there, Big can be seen to the left, hanging onto the wall further down the shaft.
Chao Boxes locations
In the room following the first point marker, take the lift on the player's right to reach the ledge where the first box is.
In the Level 3 of the stage, amongest the wooden containers in a side ledge, located just before the third Point Marker and Level 4 door.
The third Chao Box is located in the room where the Goal Ring is.