
The center of the Old Christian Salona is located in the northwestern part of the eastern city extension. A representative episcopal center with double basilicas, a baptistery and the bishop's palace was built on this area in the 5th century.
North of the Old Christian center, near the city walls, there is a building that was built at the end of the 3rd or the beginning of the 4th century. Ejnar Dyggve called it oratory A, considering it the oldest Salonitan church, which was located in the private house of a rich Salonitan, where Salonitan Christians met in secret before the year 313. Some researchers believe that the oratory was built in the middle of the 3rd century, at the time of Venantius, the first Salonitan bishop and martyr. The building consists of a tiled central hall with a semicircular stone bench for the clergy, which was separated from the area for the faithful by an altar partition. The main entrance to the hall was in the west, from the courtyard with the fountain and the canteen. Recent audit research questions Dyggve's interpretation of the purpose of this building as a domus ecclesiae. To the south of that building, next to the city walls, there are the ruins of a smaller building which is considered to be Oratory B.
The complex of twin, interconnected basilicas consisted of two longitudinal monumental churches that shared a covered vestibule, the narthex, in the west. The northern one (Basilica urbana), dedicated to Christ, and later to the Blessed Virgin Mary, had a congregational function, while the southern one, as a memorial church, was dedicated to local patrons.
The Basilica Urbana, which is also called the Salonitan Cathedral, is the largest Dalmatian basilica. Massive columns made of Brač stone with Corinthian capitals divided its interior into three naves. The presbytery of the basilica included an apse and a square space surrounded by an altar partition made of marble columns decorated with a shell motif. In the sanctuary there was a semicircular bench for the clergy with a lectern for the bishop, and in its center stood an altar with a ciborium. Along the wall of the apse there was also a bench for the lower clergy with a corridor. A famous mosaic inscription about the construction was found in its floor, from which it is known that the construction of the church was started by Bishop Simferije and completed by Bishop Hezichije. The inscription is kept in the lapidary of the Archaeological Museum in Split. Next to the northern corner of the church was the prothesis - a room where sacrificial gifts, bread and wine, were prepared for the mass. Diaconicon, a room where liturgical books, church utensils and priest's vestments were kept, was located at the bottom of the south nave, next to the presbytery. The interior of the basilica was richly decorated with multicolored geometrically shaped mosaics that covered the floors. There were colorful fresco paintings on the walls, while the walls of the apse were lined with marble.
During the remodeling of the episcopal center in the first half of the 6th century, a new monumental church in the shape of a Greek cross was built on the site of the southern, longitudinal basilica. The interior of that basilica was also richly ornamented. The columns and plutei of the altar partition are decorated with carved monograms of Archbishop Honorius II, and the floors are completely decorated with mosaics. The cross basilica was connected with the northern one by a door that led from the northern arm of the cross. Two councils of all the bishops of Dalmatia were held in that newly built basilica, created under the Byzantine influence of Justinian's time, convened in 530 and 533 by Archbishop Honorius II.
To the north of the Salonitan cathedral was a rectangular baptistery with a hexagonal baptismal font lined with marble slabs. To the west of the baptistery there is a catechumen, a hall for religious instruction, and to the east auxiliary rooms: a waiting room and a dressing room. In the 6th century, the entire baptistery complex was remodeled so that a new, octagonal one with a cross well in the middle was built on the site of the former rectangular building. Inside, the baptismal hall was circular in shape, divided by semicircular niches and massive marble columns. The interior of the baptistery was decorated with frescoes and golden mosaics, and the floor was paved with multi-colored marble. The baptism ceremony performed by the bishop was an extremely significant moment in the life of every Christian, as well as his community. This holy act was preceded by religious instruction and preparation for the rite of baptism, which took place in the catechumenium, a square room with a semicircular bench and a floor decorated with mosaics. The figural depiction of two deer drinking water from a bucket accompanied by the words from Psalm 42 is especially noteworthy: SICVT CERVVS DESIDERAT AD FONTES AQVARUM, ITA DESIDERAT ANIMA MEA AD TE DEVS ("As a hind longs for a spring-water, so my soul longs, O God, for You”), as a symbol of baptism. Unfortunately, this mosaic disappeared in the last century, but its appearance and inscription have been preserved in the drawing. After the rite of baptism, the newly baptized entered the Salonitan first church for the first time from the baptistery through the open vestibule. The lobby was decorated with marble columns with capitals decorated with figures of birds of prey.
To the northeast of the basilica complex are the ruins of the bishop's palace, episkopi. The building consisted of a large number of rooms, which were preserved in the foundations. The residential part was located on the first floor, while some rooms on the ground floor also had an economic purpose. The main entrance was located on the eastern side of the building, in the so-called Petrova Street, which was named after the Salonitan bishop Peter (554 - 562). He had a monumental ciborium set up on high bases with marble columns ending in capitals engraved with his monograms in front of the main entrance.