You could think of this space as landlocked by surrounding buildings, exposed to the environment vertically but having no (intended) connections to paths or streets. The only ways to enter and exit would be from doors and access ways of previously mentioned buildings.
Researching this term is difficult as it contradicts the definition of an alley(way), which was my initial term.
Further Information:
This is mostly a space behind or within clusters of buildings in which an open air area is shared as a means of transportation, excluded from the access and view of pedestrians/civilians/shoppers (unintended persons), typically requiring either security credentials or permission of some kind.
This is in the context of the modern world, I have experienced it behind small business malls and between clusters of laboratories, but am unaware of a way to specifically refer to this space.
To clarify even further, this isn't as literal as being completely blocked off on all sides by adjoining buildings. Natural terrain, fences, and infrastructure could block access to it as well.
Context Within Story:
It is assumed to be used by workers and business owners, connecting the back of their store fronts in a semi-communal way to assumedly transport goods and workers. The character uses this as a means to get access to an illicit marketplace, initially entering from a legitimate store front.
Appended Progress:
Passageway and Courtyard have been brought up, and both are in the general direction of what I am trying to convey. Passageway seems off to me as it is too linear/restrictive in direction, and courtyard is off in the sense that it is too public (and perhaps large).
Something like a "hidden/private court" offers a more complete picture in my mind, but is too romantic/classical in contrast to the setting and lowly premise.