Noun a four sided enclosure usually surrounded by buildings?

The most common noun for a four-sided enclosure usually surrounded by buildings is courtyard.

Here are some other possibilities, depending on the specific context:

* Quadrangle: Often used for a courtyard within a university or college.

* Patio: Typically a paved or tiled outdoor area, often adjacent to a house.

* Plaza: A public square or open space, often surrounded by buildings.

* Atrium: A large open space, often with a skylight, within a building.