A roundhead was a type of mace used during the English Civil War

English Civil War

The English Civil War was a series of armed conflicts and political machinations between Parliamentarians and Royalists...

. It is described as having a head about nine inches long, a staff two yards long inserted into the head, twelve iron spikes round about, with another spike in the end. In 1643 an article in Mercurius Civicus

Mercurius Civicus

Mercurius Civicus: Londons Intelligencer, or, Truth impartially related from thence to the whole Kingdome to prevent mis-information was an English Civil War weekly newspaper appearing on Thursdays from 4 May 1643 to 10 December 1646 published by John Wright and Thomas Bates...

claimed the weapon was called a roundhead by the Cavalier

Cavalier

Cavalier was the name used by Parliamentarians for a Royalist supporter of King Charles I and son Charles II during the English Civil War, the Interregnum, and the Restoration...

s because they were to be used to beat the Roundhead

Roundhead

"Roundhead" was the nickname given to the supporters of the Parliament during the English Civil War. Also known as Parliamentarians, they fought against King Charles I and his supporters, the Cavaliers , who claimed absolute power and the divine right of kings...

s into subjection.