Tell me about the weird doors

Watched this for the first time yesterday. I noticed there are a lot of doors where it looks like a regular door, but instead there is a smaller hatch in the middle of the door that people open and then have to hunch over to get through. Seemed odd and I don't understand why they don't just have the full wooden door in the doorway open instead of putting a little "doggie door" type thing in the middle. Was this a common design in the period these stories come from? Why?