These little metal grills represent medieval security. In the days before home contents insurance, CCTV cameras and modern fish eye lens peep holes basic home security was a little more crude.
If someone came wiping their feet on the welcome mat and knocking on the door in the Dark Ages it probably wasn’t too advisable just to turn the lock and throw the wood back on its hinges. There was no way of knowing just who you would be inviting in. So doors had these little spy holes cut into the timber so that someone inside could open it and get reassurance before opening up. They have those little grills so that an unwelcome guest couldn’t stick a sword through and poke your eye out – a very useful feature in my opinion!
During prohibition in the 1930s in the USA the doors to illegal drinking establishments had little grills like these so that the doormen inside could be certain they were only giving admittance to gangsters and not the FBI. According to one story these places came to be called a speakeasy because when talking through the grill with a password or whatever the people inside preferred prospective customers to whisper or ‘speak easy’ so that they wouldn’t be overheard. Might be true, might not.