Cella Septichora, Pécs, Hungary
As per UNESCO WHS website:
In the 4th century, a remarkable series of decorated tombs were constructed in the cemetery of the Roman provincial town of Sopianae (modern Pécs). These are important both structurally and architecturally, since they were built as underground burial chambers with memorial chapels above the ground. The tombs are important also in artistic terms, since they are richly decorated with murals of outstanding quality depicting Christian themes.
Cella Septichora: Built in the 4th century AD, the seven-foiled ground-plan of this late-Roman building is unique in the Danube region. Descending four to six metres to the former Roman surface level, we can see the wall, which remains two metres tall in parts.