Inside the central building of the first area (upstairs), there is a large square room with a vaguely sun-shaped skylight and a soul sphere in the center. Surrounding this room is a narrower hallway. At the west end of the hallway is a wooden wall with a pair of vertical designs carved into it. Open the wall between the designs to reveal a small secret alcove containing a set of shotgun shells, a rocket, two medikits and a backpack.
After getting the yellow skull key in the "sewer" area inside the main building, return to the room where you found the blue skull key. A large area to the east has opened up, letting you get a combat armor vest. Approaching the armor causes another nearby room to open, this one containing an ambush and a switch. Flip the switch, then return outside. Head to the area north of the central building. There are two small "shacks" in the far east and west ends of the water. The eastern one requires the blue skull key to open, but the western one was opened by the switch you just pressed. Inside is a secret cache which includes a BFG.
After crossing over the bridge raised using the blue skull key, an alcove will open back on the opposite side in the wall of the central building.
Once inside the large central building, go past chaingunners' cages and turn left. There will be a corridor with a medikit (not on easy skills) and a rocket box sitting at the end. As you make your way to these supplies, a section of wall behind you will lower, releasing a baron of hell (a hell knight on easy skills). This wall lowers only once, therefore quickly run past the baron into the marble alcove he emerged from and grab a soul sphere inside. Press the demon-faced alcove wall to lower yourself and get out.
Leaving the aforementioned alcove, go east past the entry hallway with the chaingunners' cages. In the next room, turn right and look at the southwest corner that is lightened by a flickering lamp. Push the corner to open a secret cache containing a backpack.
At the very end of the map, you must enter a green-stoned den to activate a skull switch that releases the Cyberdemon/Spiderdemon and its comrades. One of the walls inside the den (the one where some Revenants come from, along the walls of the east stairs) is a fake: it hides a small room containing a megasphere.
Once you enter the water behind the door blocked by red skull bars (that are lowered by a switch that doesn't require a red skull key), you cannot get back onto the walkway surrounding the structure. If you do this without having lowered the sky wall first (by running over a trigger in front of the red skull key), you will become stuck and unable to progress any further. However, a glide can be done to reach the previous area of the map between the wood and the stone walls.
Not so much a bug as an oversight: In the end of the intermission text, it is said that the player can hear "the stomp of a Cyberdemon's iron shoe". This creates a rather humorous end to the level if played on the skill levels "1" and "2", as no Cyberdemon is present at the end - on such circumstances, instead, a Spiderdemon will replace the Cyberdemon.
According to the Text screen the player sees upon entering the map, this level does not take place on the Io moonbase like the rest of TNT: Evilution, but rather in a vacation resort on Earth (presumably the Caribbean islands).