Travel to the present, walk up to the gate, then back to the past where you've just passed through the locked door.

When their conversation is over, they'll leave. Peek through the keyhole or just listen at the door and the guards outside will tell you what you need to do in this mission - get the study door combination from Stilton, who is out back. You're going to be stuck in here, but you need to hear the guards talking outside. Try out the Timepiece inside Stilton's room. Or you can just straight-up travel to that night - though nothing you're carrying (like a body or a bottle) comes with you. You can use the lens to view, in real time, what is happening in the past. The Timepiece links you to the night three years ago when the Duke's plan started. The Outsider will appear and give you the Timepiece. The floor is rotted out and you can drop in to chat with him.

Turn left from the stairs and go down the hallway to find the room over Stilton.

There's an easy to find rune in the room in front of you at the top of the stairs.

Dishonored 2 mission 7 safes