Once you arrive inside the Borgo del Castello you need to enter the innermost part, directly overlooking the sea to reach the Rocca square and be able to see what emerged thanks to the excavations of the early Christian church.

The currently visible part is located in the left coner of the square, under a canopy positioned to protect the structures.

The church, divided into three naves, dates back to the 5th-6th century. It was originally a partly underground building that rested on the polygonal walls located beneath the building called Casa della legnaia. In the excavated part, about half of the church, the apse is visible, with traces of frescoes, fragments of columns still standing and the baptismal font in the right nave. The church, in use until about the middle of the 14th century, was abandoned perhaps following a fire and then, after the stripping of the most valuable materials, completely buried and forgotten. Excavations around this area have led to the discovery of a large medieval cemetery with hundreds of burials. Studies conducted by the Department of Biology of the University of Tor Vergata have allowed us to reconstruct a very interesting cross-section of the castle's population between the 13th and 14th centuries. The materials found during the excavation are on display in the "Museo del Castello di Santa Severa" set up inside the Rocca.