The famous Scottish architect Robert Adam considered this temple one of the most beautiful European monuments. Rectangular floor plan, the temple served to celebrate the cult of Jupiter. It lies on an elevated podium, and in front of the temple stood a porch on six pillars. the vault influenced the early Renaissance architecture of Andrija Aleši and Nikola Firentinac in Trogir.
The transformation into the Baptistery took place in late antiquity, and a crypt dedicated to St. John the Baptist was built under the building. Tommy. At the beginning of the 13th century, a baptismal font was erected in the former temple, consisting of a pluteus of the altar partition (11th century), originally located in the cathedral. On one of the plaques there is a depiction of a Croatian king (Petar Kreshimir IV. Or Zvonimir), the earliest depiction of a European king in a medieval stone sculpture.
Today, the Baptistery is dominated by the Art Nouveau sculpture of St. John the Baptist, after whom the temple is named after the transformation, the work of Ivan Meshtrovich, and in front of it is placed one of the few completely or partially preserved granite sphinxes that Diocletian brought from Egypt. The font is available to visitors today, along with a ticket.