How do I get into Overseer Campbell's secret area?

When it gives you the options for the glasses, choose to spill them then wait and they will walk down there and Campbell will open it for you. But make sure if you're trying to save the captain to choke him before he gets down there, or Campbell will kill him.

I found it shortly after entering the building through the drainage pipe. Directly in front of the building, there is a pit with a pipe you can get into and follow into the lower level. When you enter the building, there will be a door in the same room you just entered, which leads to the kennel. Go through the kennel to the door on the other side. After exiting the kennel through that door, Holger's statue will be right next to you.

There are other, simpler ways to reach the sanctum, but that's just how I found it.

As you enter the building go to the right where the stairs are before going up the stairs look to the left their is a door. Go in that door follow the stairs down on the left will be the door to the Hounds on the right in the corner should be the statue Using the dark vision should make the eye blue.

Go to "the kennals" go to the very end and you will get into the basement of cambells estate, there is a bust in the room with a funky looking eye.... press the eye and there you go.

If you're upstairs, go down the east stairs. At the bottom is a door on the north wall. If you look at the map at the top of the stairs it's listed as #6. Go down the stairs and through another door. There you'll find a door to the kennels. Don't go there. Look around the room until you find a bust with a sparkly eye and give it a poke in that eye. A sealed door will open leading to Campbell's Secret Chamber.
OR
If you're in the backyard, go up the stairs near the alarm to a well lit room. Go through the door there, #5 on the map, and horseshoe around to your left and go through the next door, #6. Go down the stairs and through the door. There you'll find a door to the kennels. Don't go there. Look around the room until you find a bust with a sparkly eye and give it a poke in that eye. A sealed door will open leading to Campbell's Secret Chamber.
OR
If you're in the courtyard in front of the building, go to the south/east corner to find a door and window. Go through either to find two more doors, one leading to the sewers and one to the kennels. Take the door to the kennels and go to the other exit (not the locked door, combination 217) at the end of the hall. This will take you to the room at the bottom of the stairs leading from door #6. Don't go that way. Look around for a bust with a shiny eye and push the eye to access the Secret Chamber.
Or
If you're in front of the building but on the street outside the courtyard, there's a sewer entrance south and across the street of the west wing. This will take you to a room that has two more doors, one to the courtyard and one to the kennels. Take the door to the kennels and go to the other exit (not the locked door, combination 217) at the end of the hall. This will take you to the room at the bottom of the stairs leading from door #6. Don't go that way. Look around for a bust with a shiny eye and push the eye to access the Secret Chamber.