The Cathedral stands in the nucleus of Koper and dominates the area between the east side of the main square, Tito Square (Titov trg), and Brolo Square (trg Brollo). The three-nave Romanesque space with three apses was built in the second half of the 12th century. The Romanesque construction is especially preserved on the south side, where typical funnel-shape windows are intact and the stonework is imitated in the facade. Towards the west the church was extended to the bell tower and in 1392 it was Gothicized. The lower floor facade by the square has remained Gothic. The upper floor was redone with pilaster separations after the fire in 1460. The decorative Renaissance elements are the most distinctive on the west side, built in 1488, and in the chiselled details, such as the portals.

In the middle of the square, right next to the west façade, stands a mighty self-supporting bell tower repaired as a city tower in the 15th century. It has four main floors. The upper is open on all sides by quadra-forums. Higher up is a terrace and an ending with an Aquileian cap. In the bell tower hangs one of the oldest bells in Slovenia (1333). It was cast by Master Jakob in Venice. The upper terrace is periodically open and offers a great view of the Bay of Trieste. The clock, positioned in the middle of the third floor, faces the square