Entrance for a Hobbit
The massive, spherical cutout was, indeed, a door. It goes to the hallway, as you might imagine. However, it is unclear why the homeowners chose to install a round door like this one. Not to add that the door entry is not flush with the floor, posing a trip hazard.
Does this imply that the left-hand double doors lead to a cabinet? It appears to be an unusual method of entering and exiting a room. The spherical door appears to be modeled after the vault door.