That is a Porters Squint.
The manor house doors would be closed at night. If somebody came during the night demanding entrance it would have been dangerous to open them. The Porters Squint allowed a porter to demand some documents be passed through the slit to prove the visitors identity.
The photograph below shows the Porters Squint from inside the door. I am not sure why the grill is there. The external slit is far too small for anybody to squirm their way in. Maybe the idea was to discourage porters from being tempted to try and peer through the slit to see who was outside.