Thanks for the pictures! Are you sure the “chapel” wasn’t a baptistry? It is eight-sided, a number associated with baptism. The early Christian baptistries at Ravenna have that shape.
Thanks for the information, Anne. I tried to do some more research to no avail. Could be a baptistery. It looks like part of Budapest Castle’s older, medieval section. Half of that tower piece is gone though they covered it from the elements. Because it’s eight sided and has church-style windows you could very well be right.