As soon as you begin this level you'll have to battle it out against a Supersoldier, only this one can roam around freely, meaning you have to kill it the hard way. Luckily, you have your guns, so put some rounds into it and stay on the move. When the beast drops, take out the three or so advancing Nazi soldiers. This will allow you to explore the area behind the stairs and to the sides.
It's the area behind the stairs that you're looking for, however, as your objective is a lever that sits on the desk. You'll also find a large cache of weapons, ammunition, health and armor, and you'll want to grab up as much as possible before continuing.
Pull the lever to open the large iron gate above you, then head back to the main foyer and begin working through the waves of Nazis. You can do this from where you entered the Wolfenstein Keep, but you will eventually want to make your way to the third level, just above the main entrance. It's there that you'll find a Heavy Machinegun, and that's going to be key to taking down the Heavy Soldiers that are inbound.
Tip: Don't waste the ammunition from your Heavy Machinegun on regular guards. Save it for the Heavy Soldiers that attack in the second and third waves.
Still on the third level, look for a hatch in the wall that you can pry open with your Pipes. This will bring you to a wall you can climb, and doing so will end up with two new Signal Detected notifications on your screen. These ones aren't too bad, but in the interest of full disclosure, we messed up and went guns blazing.
You'll want to try and maintain stealth, however, and the first Commander walks a path that brings him to the end of the hallway that's on the opposite side of the room from your perch. It's a long shot to pull off with a suppressed Handgun, so we opted to take out the soldier directly below us, drop to the ground and cut him off. You can then follow that hallway, through the door with the two white aprons hanging on its right side, putting yourself in line to take out the second Commander. No matter what, make sure the second Commander falls before he sounds an alarm.
When both Commanders are down, work through the area and kill any Heavy Soldiers or Nazis that are still breathing. Be sure to loot the area for all it's worth, then climb up to the overhead beams and pry open the hatch with "K/15" marked on the side. This will put you in a crawl space, and when you come out at the other end you'll be back in what actually looks like a castle corridor.
Look to your left, down the hallway to lone guard. Note that he has a dog next to him, so make your suppressed Handgun round count, killing the human instantly. Ignore the dog for now and sneak into the first room on the right, performing a takedown on the man inside. Sneak down the hallway and do the same to the dog, then enter the room beside him and silently eliminate the last Nazi in this area. Loot the room, then use your Pipes to break the wall inside the fireplace and progress forward.
You're now faced with two more Signal Detected notifications, and the path forward is fairly linear for a moment. Sneak to the balcony and watch the two guards at the end, allowing them to split up before you end both of their lives.
As you move forward you will come to a dining hall, and the first Commander should be hanging out on the lower level. The second one likes to stay up high, and you'll find that your objective is up the stairs and to the left. It's a small hatch that you can pry open with your Pipes to move to the following area. If things go horribly wrong, just kill all the bad guys, pick up all the loot, and continue on your journey.
When you're through the hatch, hop up on the boxes and smash through the wall with your Pipes. You'll find yourself on another balcony, only this time you have a couple snipers to contend with on the right side. Take them down (just go loud, there are no Commanders here... yet), as well as any other soldiers that might be shooting at you.
Move up the stairs to the first landing, then use the small iron hatch to sneak into the library. You should get two Signal Detected messages now, but you can get close to both of the Commanders by sneaking up the nearby stairwell. You just have to watch out for Nazis, including the Heavy Soldier, who refuses to be stealth killed (unless we missed something).
Exit the second floor of the library onto a balcony and hang a left. This will take you to a series of hallways where you might also find the Commanders. If you've set off the alarm, they will almost certainly be hiding out here. When you reach the end of that path you'll have to Sprint Slide under one of the doorways, completing your objective.
Wolfenstein Keep | Save Wesley
Your first instinct will be to run to your friend and free him, but we'd strongly advise that you don't. Instead, spend a few moments getting very familiar with the area, including where all of the health, armor and ammunition is located. Do not pick it up, but spend some time getting to know where you can find each of the three. After you interact with Wesley, you're going to want to know where all of this is for the upcoming battle.
When you are familiar with the layout of the area, go to your friend. This will kick off some face time with your old pal, Rudi Jäger, and there's not much to do beyond following the on-screen prompts, then stabbing his dog to death when the time comes. It's after this that the hard part begins.
Head upstairs and gather up all the supplies you can carry. You could go to the far back stairwells, but this is a tough place to find health, and you can still be attacked from three angles. We actually opted to fight from upstairs, but behind the bar. It allowed us to duck when need be, popping up and engaging enemies when the time was right.
However you decide to fight, it's a tough one. When it's over, look for your on-screen objective, moving to it and pushing the button to open the door. When you pull the lever for the elevator, the chapter will be over.