In the end I didn’t actually not kill anyone. I have no idea who I killed, but the quest summary said I killed 2 enemies. But I still had a low chaos rating so the 2 kills only really impact my pride. Anyway, this guide explains how to eliminate High Overseer Campbell non lethally and indeed all enemies non lethally and without being detected. There are a few secrets in this area, I also talk about where those are located.
There is a video at the end of this guide containing a visual version of this guide.
When you load into the area you see a guard with his back to you. Just choke him out and free the prisoner. That is one of the optional parts of the quest. Next you have to get over the gate. To the left of it there is a crevice with a dumpster and some pipes. Jump on those until you are above the gate:
There are two enemies below you. Jump down and hide behind the train until the one closest to walks away from you. Use your teleport ability to get close to him and choke him out. Be sure to hide him somewhere behind the train. The other enemy is alone, just approach him carefully and take him out from behind at an opportune moment.
Next you get a guard house:
The guard in there eventually goes behind his desk and studies some shelves thoroughly giving you the chance to slip by or choke him out. The latter will easily allow you to loot the area. When you exit the guardhouse you get information about the fact that you can choose several different paths. I chose the one that leads directly down in front of the door, which is actually the sewer entrance. Just follow the pipes until you get to the Kennels:
Unfortunately in this area I had to take a break so I don’t properly remember what I did. This is also the area where I suspect I killed 2 guards by releasing some hounds, so be sure to avoid doing that. There is actually a secret stash in the kennels, I have written a guide for getting to it.
The best advice I can give you for the Kennels is to use the pipes near the ceiling. You only have to drop down once, use your teleport ability to get to the next pipes quickly:
Guards don’t look up too much in Dishonored, so verticality is very powerful in this game. Follow the pipes to the end and you will see a door on the opposite wall. Use your teleport to get there and immediately press the action button to get to the next area.
Before you exit the stairway:
Wait until a guard walks past and take him out. There is another guard on the stairs to the left. Wait until he walks up the stairs and take him out from behind. There are two guards in the stairwell on the opposite side of the bottom floor and 2 guards in the central area. Take them out one by one whilst utilizing the teleportation skill.
It is best to use the stairs near the Kennels to go up to the next floor. When you do there are a number of guards around the place:
Take them out one by one from behind whilst making sure you hide their bodies.
If you want to complete this quest non lethally you have to search the archives for some information. There is a guard up top of the archives:
But he has his back turned so you can easily take him out. Be careful though, after a while more guards entered the archives for me. You will also have to go through the interrogation rooms to find the Heretic’s Brand:
You are now ready to take out High Overseer Campbell. But before you do you should loot Campbell’s secret room, I wrote a guide for that too.
Enter the meeting room any way you like. As soon as you do Campbell and an officer will enter the room after a certain time too. be sure to pick up the run and to spill the wine to make sure Curnow survives:
When Campbell and Curnow leave the room follow them. When they are just outside grab Curnow from behind and choke him out. Campbell won’t notice anything. You have to bring Curnow’s body to a dumpster. Step through a window onto a ledge:
And follow that ledge until you are above the dumpster. You can actually use the teleport ability whilst you are carrying a body. Don’t drop Curnow because he is going to die if you do. Use your teleport ability to get into the dumpster.
Getting back into the building is actually somewhat difficult because all the guards in this area are still around. Stay behind the dumpster until you can use your teleport to get into the open gate undetected:
Then jump on the boxes to the left. Look up to the pipes and use your teleport ability to get onto them and then on a ledge. Follow that ledge until you are back inside the building.
3 guards enter the area. Be sure to keep an eye out for them and take them out when you find them. Again utilize your teleport ability to get behind them. High Overseer Campbell is in his secret room. Go down there and choke him out. Now you have to take him into the interrogation room. This will be difficult if there are any guards around, so be sure to check around the place to make sure there aren’t. No more guards will spawn after the 3 mentioned earlier.
After you have branded campbell you have to exit the area through the back yard. In the back yard there are several bone charms and runes, but describing how to get to them wouldn’t make much sense. If you have any trouble check the video at the end of this guide, I show how to get all of them, and a safe, around the 5 minute mark.