In brief
Early Christian architecture organized different levels of a complex.
Useful to know
In the necropolis of ancient Sopiane, present-day Pécs, decorated tombs were built in the fourth century. UNESCO points to underground burial chambers with memorial chapels above ground and murals with Christian themes. [1]
The cross-section here can explain more than a general external view. Different levels have related but not identical functions. When considered, separate the structure from the painting program: both are important, but give different information. It is not enough to say "Roman ruins" when the particular spatial solution is known.
Sources
For further detail and to check the information: