This is the pre-Romanesque church of the Holy Trinity, located in the suburbs of the city of Split in modern Croatia. The exact date of its construction is not recorded but its architecture indicates a period between the 8th and 11th centuries. While most tourists visiting Split today are understandably interested in the spectacular ruins of the roman palace constructed there for the emperor Diocletian in the late 3rd and early 4th century (today on UNESCO list) I think this small structure really adds to the experience of the site. It shows a period of transition. By that time the enormous Roman palace was already destroyed but its ruined walls were still inhabited by Roman or Greek refugees clinging to what was left of the Roman imperial heritage and Byzantine trade. Just outside the massive walls of the Roman palace however, the new type of structures already began to appear signalling the formation of a new, early medieval state dominated by Slavs who settled in the region at the end of the barbarian migrations.
Ps. Similar rotundas were frequently constructed in other early medieval states north of Croatia, for example in Great Moravia or later in Poland or Bohemia