Enter the open hallway and you'll come to a watery chamber. Climb the beams, shoot the barrels for some freebies, then push the lever to raise the bars in the other room.

Shoot the barrels in the water, dive into grab some more supplies minding the Devil Fish - then return to the other room and enter the newly opened passage. Walk toward the alcove on your right and a lift will lower.

Go up, kill the Krall and turn the capstan to open the main doors below. Quickly get down and go through the doors before they close. Once inside, turn left, and follow the hallway around in this direction.

Walk up the sloped support along the right-hand wall and get the Auto Mag clips on the balcony. Jump down and walk up the stairs on the right to reach a landing with a Razorjack on it.

Below the landing, there's a chamber with a Krall inside. Kill it and push the lever to open the inner doors. Enter the inner castle. The open room on the right contains supplies - turn right as you leave it, head down the corridor and up the stairs at the end.

The chamber at the top is a trap. There's a Titan inside and, as soon as you enter the door will lock behind you. Krall will also swarm into the room. Push the loose block on the back side of the central pillar to open a secret door run up the stairs, out onto the platform, and lob grenades down on the Titan - he'll hit you into the air which won't hurt, so get him to hit you high enough so that you fall into the cistern in the middle of the platform.

Grab the freebies inside, swim back out and help the Krall finish off the Titan. Once he's dead, don't go back to the room below. Make the jump from the platform to the small landing above the barred door push the button to open the gate below and escape the chamber Go to the new hallway on the left and push the lever. Turn around and enter either door on your right.

Climb the stairs. The levers and weights at the other end of the chamber lower a lift near the ledge - ride it up, walk along the ledge in the upper chamber push the button and jump to the platform below for a Super Health.

A secret panel underwater has an Invisibility power up inside. Grab it and reach the lever at the bottom of the pool, swim up and exit via either door. Go back downstairs and enter the room across from the stairs. The bars over the opening are now gone. Go through and down the stairs. At the bottom, bars will drop behind and Krall will attack.

Run left and enter the alcove to ride a lift up. Walk along the landing where a lever opens the bars blocking the room. Go to the pool and dive in, swim through the gaps in the bars toward the underwater shaft with no grate on it. Follow the shaft to a small chamber grab the goodies and push the lever.

Swim back up and through the bars in to the area outside. Go into the hallway, leave through the passage and turn left. Enter through the gate, walk down the stairs and push the lever to open the doors across the hall.

Watch out for the hordes of beasts that'll attack you and turn the wheel to lower twin lifts to the upper level. Go up, take the stairway behind the lifts to a room full of crucified Nali and grab the Healing Fruit. Head the other way from the lift, through a passage - beware the ceiling-attack Krall.

When they're all dead, the gate will open. Go through and push another lever to bring a lift down. Ride it up, push a lever and go through the passage behind the lift where a panel just opened.