A fascinating architectural detail they used to do often. I had some fun doing this one in a bathroom I built in our old house. It's a major heavy-thick(fire protection and sound deadening) 3' wide door for moving big items through to a studio(sometimes seasonally rented). But it's usually closed for the bathroom and laundry room.

When you need to go through, turn the black door knob. A house should be fun to live in.