What is a "rotunda"?
A rotunda (from Latin 'rotundus') is any building with a circular ground plan, and sometimes covered by a dome. It may also refer to a round room within a building (a famous example being the one below the dome of the United States Capitol in Washington, D.C.). The Pantheon in Rome is a famous rotunda. A 'band rotunda' is a circular bandstand, usually with a dome.
A great number of parochial churches were built in this form in the 9th to 11th centuries CE in Central Europe. These round churches can be found in great number in Hungary, Poland, Slovakia, Croatia (particularly Dalmatia) Austria, Bavaria, Germany, and the Czech Republic. It was thought of as a structure descending from the Roman Pantheon. However, it can be found mainly not on former Roman territories, but in Central Europe. Generally its size was 6–9 meters inner diameter and the apse was directed toward the east. Sometimes three or four apses were attached to the central circle and this type has relatives even in the Caucasus.