The cavaliers were against the roundheads in the civil war that was in the 1640's. The cavaliers were on King Charles I's side whilst the roundheads were on Oliver Cromwell and the parliament's side. Cromwell disagreed with all of the King's decisions and that is how the war started. Eventually, the King's head was chopped off and the executer was left un-known. In case you haven't already realised the roundheads won. Parliament was in charge of the country but not long after another King came along. Hope this was useful!