Many of the fortifications built by the Romans in England survived into the Middle Ages, including the walls surrounding their military forts and cities. These defences were often reused during the unstable post-Roman period. The Anglo-Saxon kings undertook significant planned urban expansion in the 8th and 9th centuries, creating "burhs", often protected with earth and wood ramparts. "Burh" walls sometimes utilised older Roman fortifications, both for practical reasons and to bolster their owners' reputations through the symbolism of former Roman power.