The Battle of Cassel was fought on 22 February 1071 between Robert I of Flanders (or Robert the Frisian) and his nephew, Arnulf III (son of Baldwin VI of Flanders). The battle was a victory for Robert I of Flanders, Arnulf III was killed in the battle. Robert's forces attacked Arnulf's numerically superior army before it could organize. Arnulf himself was killed along with William FitzOsborn while Richilde was captured by Robert's forces. However, Robert himself was also captured by Eustace II. Ultimately, Richilde was exchanged for Robert's freedom.