Warfare in Normandy at the time was shaped by the defensive potential of castles and the increasing costs of conducting campaigns. The Norman frontiers had limited natural defences but were heavily reinforced with castles, such as Château Gaillard, at strategic points, built and maintained at considerable expense. It was difficult for a commander to advance far into fresh territory without having secured his lines of communication by capturing these fortifications, which slowed the progress of any attack. Armies of the period could be formed from either feudal or mercenary forces. Feudal levies could only be raised for a fixed length of time before they returned home, forcing an end to a campaign; mercenary forces, often called Brabançons after the Duchy of Brabant but actually recruited from across northern Europe, could operate all year long and provide a commander with more strategic options to pursue a campaign, but cost much more than equivalent feudal forces. As a result, commanders of the period were increasingly drawing on larger numbers of mercenaries.