The current church was supposedly built in the 11th century and since then underwent numerous renovations. The bottom part was designed in the Gothic style, while the upper part appears to be in the Renaissance style. The cathedral was expanded in the first half of the 18th century. Nowadays, the church is famous as one of the largest churches in Slovenia. It measures 70 metres in length and 25 metres in width.
Inside the church, you can admire the artworks by Vittore Carpaccio and the marble headstone of St Nazarius, the patron saint of the city and the bishop of Koper (from the 6th century), which was gifted to the city by the Republic of Venice around 1350.
The unique feature of the cathedral of Koper is the baptistery – the Rotunda of Our Lady of Mount Carmel (formerly known as the baptistery of St John the Baptist), which is separated from the church. It is a Romanesque rotunda with a permanently blind arch. Inside is a baptismal font. The baptistery was reconstructed in 1317, and its inside was later designed in the Baroque style (1747–1749).
Beside the church is a 54 metres high Bell Tower from the 12th century, which was joined to the cathedral in 1488.
The cathedral also has a magnificent 4 manual symphonic organ, which is the largest church organ in Slovenia. It was made in 1988 by the German organ workshops Kleuker and Steinmeyer according to the plans of the French composer, organist, pianist and pedagogue Jean Guillo (1930-2019).