The cathedral is situated in a complex of Diocletian's Palace at the Peristyle and was originally an imperial mausoleum built in the early 4th century cathedral, and it is the oldest cathedral in the world. The building's exterior is octagonal shape. Portal of the cathedral is also an ancient origin. The bell tower of the Cathedral of St. Duje is one of the most original church towers along the Adriatic coast. The construction ot the bell-tower began in the mid-13th century and lasted until about the mid-16th century. Due to the extremely long period of construction, a combination of Romanesque and Gothic architecture, but both styles are in excellent agreement. In the decorative elements Romanesque style dominates, and the architectural design of clarity belongs to the Gothic architectural style. The bell tower is a distinctive architectural work, specifically for its slenderness and transparency, by narrowing down the staircase to the top, and fitting in antique architectural environment, because the use of cornices and capitals, in the format of openings and arches, arcades corresponds to the Peristyle.