Doors of a Staircase/Guard house/Garden:
The door of a staircase, guard house or garden is obligated to have a Mezuzah if the area leads to an opening which is obligated to have a Mezuzah. Thus the doors of stairways which lead to apartments are obligated to have a Mezuzah. Similarly, hallways that contain doorways are obligated to have a Mezuzah. If however the staircase/guard house/garden does not lead to a dwelling area that is obligated in a Mezuzah then it is exempt from having a Mezuzah. However there are opinions which obligate a guard house [and garden] to have a Mezuzah even if it does not lead to a house of dwelling.