It's like a regular front door on each apartment, except on the inside and outside there is a bubble, or half cylinder.
The girl I was with said that they were so the residences could put their coats inside the door and the maids or whoever would open up the outside of the door and clean the coats.
I found this very interesting.
But as always I was slightly skeptical, so I tried to look it up online. But I didn't know where to start, so I got nowhere.
If you know, share please!