The baptistery was declared a national monument in 1931 and is the only one of its type to exist in Spain. This underground chamber dates back to the 3rd century AD. It has a long vaulted corridor that was a mausoleum and a small room covered by a cupola. Here there is a side altar and a spiral staircase in one corner.
The octagonal nymphaeum, a small pool in which to perform the rite of baptism, was located in the centre of the quadrangular room.