To get the red card involves several steps. First, make your way to the big nukage pool through the left blue door as you are facing the pair of blue doors. Immediately to your left is a nukage hallway. Go straight through, and upon making a right turn, go straight to where the spiked soldier hangs. Turn right, then right again to a lift, which takes you to a raised platform with a yellow card switch. That switch lowers bars in the room where the 6 cans on fire are. There is a switch now exposed, when hit, will lower the teleporter platform next to it. Be careful, entering the teleporter puts you on the same platform as the red card, a soul sphere, a chain gunner and 4 barrons. Best strategy is to jump off the platform, and hit them from afar with rockets.
Behind the blue doors is a huge nukage pool. At the back of this area is a tunnel, at the start of which you can see a rocket launcher behind a gate. Continue along the tunnel to the right, hugging the left wall as you go. One of the walls lacks the green glow along its bottom edge that the surrounding walls have. It can be opened, and the passage behind it leads to the aforementioned rocket launcher.
In the narrow passage leading to the rocket launcher in secret #1, there is another wall that does not have a green glow on it. Open it to reveal a teleporter to a ledge back in the nukage pool area.
Still in the nukage pool area, step onto the platform bordering the eastern of the two blue doors. Quickly open the door and hug the left wall. An alcove containing a teleporter to a secret area will be open temporarily (Note: Crossing from the nukage onto the platform or vice versa is what actually triggers the alcove to open).
Behind the red doors is a wooden structure with a switch on it, at the far side of a lake of nukage. While the "front" (west-facing) side of the switch opens the door to the exit, the "back" (east-facing) side opens a secret room in the nearby cave with the glowing red floor.
In the twisted corridor at the north side of the map are two sunken areas, one containing a Hell Knight and the other containing Chaingunners. At both entrances to this section of the corridor are lines of type 130 to raise these areas. However, the one on the east is in the middle of the corridor and can be walked around. If you subsequently fall into one of the unraised pits you can become trapped.