Entering the level dive off the bridge. To the right, near the bottom is the entry for a cave. A few goodies inside.

Climbing out of the water after secret #1, you stand next to the back door. Enter. At the top of the big stairs, jump over to the L-shaped wall, from there to the sniper stand.

Entering the front door, go left on the Y-shaped platform, and past the traps (What tr..? Ouch!), and get the blue key. Sneak up the right side of the Y-platform.

In the dark room with the single torch on a column, head-butt the torch. Unusual approach, but it works. A secret door opens behind the column.

Carry on up the stairs, to the slipgate room. If you enter the front of the slipgate, you will be sent to the gold key and almost certain death.

Squeeze in at the back of the slipgate, and you'll be sent to a secret that will help you survive the meeting with the gold key keeper. Just snatch the secret and run back into the slipgate.

When you land in front of the exit slipgate, right behind you is a misaligned bit of wall. Shoot or axe it. There is a slipgate for getting back up, and some armour.