The rotunda church of St. Donatus (the Holy Trinity) was built in the northeastern part of the Roman forum and later diocesan complex from numerous fragments of architectural elements of the Roman forum. The exceptional quality and originality of form resulted from the merging of two different projects. In the first project (end of the 8th century), the church was conceived as a free-standing rotunda, while in the second project, certainly in the period of the Bishop Donatus (the first half of the 9th century), it was constructed as a complex rotunda structure with a gallery and system of annexes around it. Valuable examples of the earliest phase of pre-Romanesque sculpture in Dalmatia come from this church. The wooden beams with carved decoration are particularly rare.

The history of the Archaeological Museum Zadar is immutably tied to the church of St. Donatus. From 1880, preservation and restoration was carried out on the church for fifteen years, and this year is considered to represent the date of the foundation of the Archaeological Museum. As early as 1886, 132 stone monuments were stored in it, and between 1893 and 1896 the other archaeological material was transferred, and the first permanent exhibition of the museum was opened in 1897. During the period of Italian rule, between 1928 and 1931, the permanent exhibition was renovated, and some of the stone monuments were displayed in a less investigated part of the Roman forum by the church, with the addition of four imperial statues from Nin that had been returned from Venice. In such a more or less unchanged state the collection awaited its transferal to the new premises of the museum in the building of the former Institute of St. Dimitri, today the University of Zadar, in 1954.