Although the sacral building has a simple architectural design, it is only on the basis of the preserved built structures that it can be deduced and assumed that it is most probably the oldest building in Koper. Its circular ground plan is clearly recognisable on the oldest map of the town, a plan drawn by Giacomo Fino in 1619 and called the 'Rotonda'. Also, the extant visitation record of the Apostolic Envoy Agostino Valier from 1580 states that the building was the Church of St Mary, popularly called "Retondi" because it was still round in shape at that time.
The reason that the central building was built for baptism is shown by the preservation of a baptismal pool in the ground under the existing floor, which was found in 1900 during renovation work. The remains of the baptismal pool have undoubtedly been detected under the floor of St Mary's Church also recently by means of a non-destructive GPR survey carried out in 2021. The building was undoubtedly built for the baptismal rite, but the method of baptism is also important for the timing of the architecture. According to experts, the turning point in the baptismal rite was the transition from adult to child baptism, which is thought to have taken place in Rome between the 6th and 9th centuries. In such a rite, the child was immersed in a baptismal pool.
The shape of the baptismal pool was usually hexagonal, deepened by three steps. It is not entirely clear when immersion baptism was abandoned, but it is very likely that it persisted in the Mediterranean until the 14th century. Koper was part of the Patriarchate of Oglele, so baptism was carried out according to the Oglele rite. In the 8th century, the rite of the Patriarchate of Oglele allowed priests to perform the rite of baptism throughout the year, and not only bishops, as had been the case before.
After the 15th century, children were baptised by pouring, so the method of baptism was completely changed, as the pool was no longer needed. Despite the efforts made so far to clarify the assumption of the original purpose of the Rotunda as a baptistery, it is possible that it was located in a part of Koper where there was relatively good access to the sea, which was used by daily traffic. Thus, the baptistery was not only intended for the baptism of the inhabitants of Koper, but also for people from elsewhere. Until 1213, children from Izola were baptised in this baptistery, as they were not allowed to have their own baptistery until then.
Experts are mostly of the opinion that the Church of St Mary was built for baptism, and that the first Christian church, perhaps even the first cathedral in Koper, could have been built in its vicinity, but to this day no such church has been found. At that time, the rule was that between the baptistery and the church there was a path to be followed. Given that another baptistery dedicated to St John the Baptist was built next to the Cathedral of Koper, the model of which is the Romanesque cathedral complex of Oglele, it is very plausible that the Rotunda of St Mary could have been part of an older church complex. The baptistery next to the Koper Cathedral dates back to the 12th century, which shows beyond any doubt that it is Mary's rotunda that confirms the assumption that there was an older baptistery in Koper.
