It was not unknown for women to ride war horses, and take their part in warfare. Joan of Arc is probably the most famous female warrior of the medieval period, but there were many others, including the Empress Matilda who, armoured and mounted, led an army against her cousin Stephen of Blois, and Stephen's wife Matilda of Boulogne in the 12th century. The 15th-century writer Christine de Pizan advised aristocratic ladies that they must "know the laws of arms and all things pertaining to warfare, ever prepared to command her men if there is need of it."