Open the Keep Gate
You're back in your boat to start this chapter, but luckily you'll have your Heavy Machinegun to keep you company. It's still going to be a rough ride, so disembark as soon as you can and take cover on the right side of the docks. From here you can defend yourself against the Nazi attack, while still enjoying some decent cover. Once the docks are clear, spend a moment grabbing up all the ammunition, health and armor that you can find. Also, don't forget to pull the lever near where you exited your boat.
When you're ready to move on, hop into the boat on the opposite side of the docks from where you entered and take a ride. Jump up on the platform and do some looting, then pull the lever and get back to your vessel. As you push forward you should get two Signal Detected notifications. Pay close attention to not only the distance, but also the direction of your targets.
Get off the boat and equip your suppressed Handgun. Take the two nearby Nazi guards down, then grab the Bombenschuss that's sitting on top of some barrels between them. If you already have this weapon, ignore us and move on. We may have missed it during an earlier chapter, and the game is kindly giving us another crack at it.
Look for a small scrawl space that will allow you to sneak into the building. If you're facing the main gate, it's just to the left. Go through this and take note of the guard, but allow the first Commander to walk directly to you. Take him out, then sneak to the nearby desk that has a painting hanging above it. There is a key on top that you'll need in order to progress to the next area.
Keep an eye on the Signal Detected so you know where the second Commander is, as he's likely to roam about a little. Work your way over to him and perform a takedown. Clear the area if you wish, or just go through the iron barred door that is marked as your objective. It sits next to a statue that's holding a sword.
When you get to the next area, equip your suppressed Handgun and drop through the hole in the floor. Crawl forward and drop the two chatting soldiers with one bullet to the brain each. If you're quick you won't be detected. Continue to crouch and move forward, gathering up supplies and putting bullets into the heads of any patrolling Nazis you find. In total there will be five humans and one dog that need to die, but we sailed through this part undetected.
After using your Pipes to pry open the floor hatch, you'll find yourself going for a swim. Crawl out of the water and climb the nearby wall. Shoot the lone soldier in the head, then turn to the left and go up some steps. Quietly kill the other soldier waiting at the top. Use his dead body as an indication of where to go, following path and silently murdering the two Nazis who are roaming about in the next area.
The second of the two enemy soldiers will be looking over a railing. When he falls, turn left and go into the small cave, following it as it curls to the right, then switches to the left once you start to ascend. When you reach the top, look to your left and take note of the Nazi standing guard.
Tip: These catacombs are not exactly easy to describe, so make sure to use your in-game map if you're lost. Also remember that we're helping you beat the level, not explore it. Wander around if you'd like.
Be careful killing the soldier we spoke about above, as he has a buddy who's standing on the other side of the broken bridge. Take them both down, then spend a moment learning about how to Sprint Jump. You'll need to do this in order to get to the other side.
When you're safely across, go up the stairs on the right, walk to the end of the wooden platform, then duck into the tunnel and follow it until you reach a wall. Climb the wall, then move forward and allow the lone guard to pass your door. Exit the room and perform a takedown on him before stepping into the elevator and prying open the roof hatch.
Climb up the wall of the elevator shaft and quickly shoot the nearby guard in the head. You'll now have two more Signal Detected notifications, and if you turn to your right just past the wooden barrels you're at, you'll be headed straight for the first one. Just be sure to kill the two Nazi soldiers that are wandering in the corridor outside of his hallway before you sneak in and take his life. If you continue to follow that same hallway where you found the first Commander, you should run into the second. He likes to patrol the wooden platform above where you entered this area.
When the Signal Detected notifications are down and the area is clear, move back to the corridor where you took down the two Nazi guards and the first Commander. There is a door that you'll need to Sprint Slide under in order to move on. Do this, then snag the Bombenschuss Scope that's sitting just shy of the iron barred catwalk.
From the doorway to the catwalk you'll have to kill a sniper. He's at your one o'clock, higher up than you and on a platform. A single bullet should do the trick, at which point you can follow the catwalk to the giant lever and open part of the Wolfenstein Keep gate.
Use the spiral stairwell to get back outside and on the ground level, but be ready to deal with a sniper at your 10 o'clock high, as well as some advancing soldiers on the ground. Try to nail the sharpshooter as quickly as possible, then switch to your Assault Rifle and clean up the remaining nonsense.
Grab any goodies that were dropped and head to the opposite side of the docks. Hop in the water and swim toward a giant opening. You should see a sign to the right of the opening, just below where you killed the last sniper, that says "Munition" and points to it. That's where you want to go.
Follow the only possible path as is curls to the left and you ascend to a loose pile of bricks that are pretending to be a proper wall. Smash through it with your Pipes, travel along the cliff, then climb the wall to put yourself in a window. You should immediately see a Heavy Soldier and a regular Nazi guard. Go for the big guy first, then take out the guard. There's no penalty for noise here, as there aren't any Commanders roaming around.
Snag the Shockhammer from the Heavy Soldier, then access your inventory and set things up so you can quick-swap between that and your Assault Rifle. You'll have a bit of a fight coming up, and the formula that worked for us was the Assault Rifle for regular Nazis, and the Shockhammer for the Heavy Soldiers.
Make your way out of the room and to the catwalk. There shouldn't be any additional opposition between you and your objective, which will be completed as soon as you pull the lever.
Enter the Lower Keep
Head down to the ground level, exiting onto the docks and looking to your right. If you have any Hand Grenades, now would be the time to use them. You might also consider using the Kampfpistole, which is a really spectacular way to obliterate your opposition. When the coast is clear, enter the Keep and crank the wheel to complete the chapter.