The name Pau means "Palisade" in Béarnais; from the 13th century there were palisades fortifying the banks of the Gave de Pau river. The castle was probably begun in the 11th century; the Mazères tower is the earliest remnant. The Montauser tower dates from the 13th century and is a square keep or donjon. The castle was expanded and fortified by Gaston III (1343-1391), Count de Foix and Viscount of Béarn; he built the 33 m high brick Monnaie tower. Henri d' Albret and his wife, Marguerite d' Angouleme, sister of François I, transformed the castle into a Renaissance chateau with a cour d'honneur and sculptural ornament beginning in 1512. King Henry IV of France and Navarre was born in the castle on December 13, 1553.