white castle was built in the wake of the norman invasion of england in 1066. shortly after the invasion, the normans pushed into the welsh marches, where william the conqueror made william fitz osbern the earl of hereford; earl william added to his new lands by capturing the towns of monmouth and chepstow. the normans used castles extensively to subdue the welsh, establish new settlements and exert their claims of lordship over the territories.