I'm going to make this walkthrough brief, this was done on expert difficultly, all loot collected, total time 26 minutes.  Please refer to the loot list for loot locations. 

Enter the fortress through the sewer outlet pipe, and open the boiler in the boiler room.  Look at the menacing flames.  The flames run in a sequence, so when the last flame has been lit about 2 seconds make a dash through the boiler, you will get a little damaged, but its really hot in there still.

Once through the boiler turn of the boiler room power station and head upstairs.  In the east end of the basement is the basement barracks and the fort prison, you can go set the pagan being held prisoner free for a distraction if needed.  If you came in quick enough you'll encounter a Sergeant with a lamp and a key to the armories.

Up the stairs is the first floor, the armory is on your right as you exit the basement door, this is a good place to hide and grab some equipment.  Now head up the stairs again to the top of the fortress wall.  Up here there are four archers and two grunts on patrol, a Sergeant makes his patrol up here, the fortress Captain comes up here, and Otto the chief engineer comes up here.  Probably best to put out some torches. To get to the next power house go right out the door, walk along the top of the wall and drop down at the power house door and head in.

Coming out the wall power station jump the wall and slide down the angled face to avoid damage, head inside and hide in the bathroom, check out the creative piece of toilet paper.  between patrols get the loot in the main barracks and the main storage room.  After the patrols are clear proceed to the main hall way and to the west end of the fort.
In the dining room there is some loot to collect and some hiding spots, when its clear head outside and across to the shop.  Inside the shop are the instructions on how to open the gate, but we're already doing that.  The shop also has some moss arrows needed next.

Back in the dining room is a secret door behind the fireplace, a moss arrow on the metal floor and its on to the kitchen.  Grab the assorted loot and the cook's key.  Out the kitchen door to the left is the basement door and the basement power station.

Head back to the kitchen, turn left and go deeper underground to the pumping room.  Here is the bridge mentioned on the toilet paper, code is 2431.  Nothing else of interest here, head back up stairs all the way to the top door and back out to the dining room.  Go all the way to the east end of the fort in the bathroom. 

Jump into the toilet, its connected to the keep bathroom.  In the keep the fourth power station is on the first floor, upstairs is the Captains quarters including his personal log and a second armory.  When done here jump out the window of the Captains room onto the ledge, moss arrow down below and drop into the shadows.

The door here is all that stands in your way, pick the lock, head into the gate house, grab some more loot and drop down the hatch.  In the basement is the last power station.  Flip the switch and the gates open up.  If you want head back upstairs and watch the fighting.  If not head down the stairs on the north side of the basement to the pumping room.  On the floor is the missing drill bit, head up the stairs to the basements.  You can go back through the basement, to the boiler room and out the sewer pipe, or all the way upstairs again to the top of the wall and jump off, either way its time to head home and collect your reward.