Battle of Cassel (1071)
Robert's forces attacked Arnulf's larger army before it could organize. Arnulf himself was killed. Also killed in the battle was William FitzOsborn. Captured were Richilde (by Robert's forces) and Robert (by Arnulf III's force). Richilde was exchanged for Robert's freedom. Robert kept the countship of Flanders and ruled a much smaller county until 1093. He gained the friendship of King Philip I of France, by offering him the hand in marriage of his stepdaughter, Bertha of Holland.