Jansen's portion of this map was originally intended for a project of his own, and was designed in 1998. The castle area in the original map was replaced by the island fortress, which was originally a standalone map designed by Johnsen in 1996.
In the first tunnel you pass through, you need to press a skull-faced switch to clear some rocks blocking a further passage in the tunnel. If you look on the backside of the skull switch, you'll see a second switch. Press it to open a gate blocking access to a rocket launcher in the room adjacent to the switch.
After emerging from the first tunnel, you'll be able to see out into the "ocean". In this area you'll see a bunch of armor bonuses lined up against the wall. Open the wall behind them to reveal a teleporter. Go through and you'll be teleported on top of one of the dilapidated structures in the starting area. Follow the wall you stand on around to get a berserk pack, then drop into the "well" to get the soul sphere inside.
Once you reach the big fortress at the end of the level, follow the outer wall to the northwest corner of the structure. Here you'll see a free-standing, tall, rectangular platform. It can be lowered like a lift so the goodies on top can be collected.
Make your way around to the backside of the fortress. There's an opening in the wall that you can drop down, with a blue teleporter at the bottom. Step through it to be taken to a raised platform at the southwest corner of the fortress, similar to the one from secret 3.
From secret 4, look at the fortress' south outer wall. You can see an extremely narrow ledge on it at the same level you're at. Follow the ledge along to the parapet at the far side. Use the gargoyle switch to lower a lift, letting you access the inside of the southeast turret.
Once inside the fortress's courtyard, open the baron-faced wall to get inside the fortress itself. Inside, turn right immediately. You can see a small red arrow on the floor pointing to the wall nearby. Open this wall to get a soul sphere.
A bit farther on in the fortress, you come across a room with three bookcases in it. One of them lowers, revealing a chaingun and a hidden room beyond.
Inside secret 7, press the gargoyle-faced switch to lower a nearby lift holding a combat armor vest.
Keep going through the fortress until you get to a room with a black-and-white tiled floor. There are two alcoves with rockets in them here; the back wall of the left-hand alcove opens, revealing a teleporter. Step through and you'll arrive on top of the northern parapet of the fortress. Like secret 5, use the gargoyle switch to lower a lift leading inside the nearby turret.