Remains of a Roman military amphitheater in Budapest, Hungary

Here is a section of a map from 1823, it shows the buildings built on top of that, note the circular shape:

Same map browsable here, change the opacity on the top right to see current aerial photo.

According to some legends it was used as a fort in the middle ages, bu afaik no archeological remains support this.

Another map from 1878, with more details, it shows the contours of the buildings:

Source