The Mishna on today’s daf teaches that all of the people living together in homes that face a courtyard must contribute to basic needs of the courtyard – e.g. a gatehouse and door. Similarly, all of the inhabitants of a city must contribute to the communal needs of the city.
The beit sha’ar, or gatehouse, was a small building that was erected at the entrance to a courtyard whose purpose was to regulate entry into the courtyard, occasionally with the assistance of a guard who would be stationed within.
Although the Mishna presents the idea of a gatehouse as essential for the courtyard, the Gemara questions whether such a structure is a good thing to have at the entrance. The Gemara tells of a certain pious individual who was often visited by Eliyahu HaNavi, and that after building a gatehouse, Elijah stopped coming to him. The Gemara offers several different possible distinctions between recommended gatehouses and those that are problematic –
Is the gatehouse positioned inside or outside the courtyard?
Does the gatehouse include a door or not?
Is the door secured by a key or not?
Can the key be accessed from the outside or only from the inside?
The idea behind all of these possibilities is that it is reasonable to construct a gatehouse that is set up to help the inhabitants of the courtyard control the flow of people coming in from the public thoroughfare, but only if it will still afford access to a poor person who wants to come and collect alms from the people who live there.
The key that is mentioned as one of the factors that is considered is called a potahat by the Gemara, a type of lock-and-key combination that was popular in the time of the Mishna. When inserted in the key hole, the door could be opened easily by a person, but it held the door closed against animals pressing on it.