Behind these ashlar blocks a core of rubble, bound with mortar, was rammed home, while wood ties framed and bound the wall until the mortar set. Sometimes, the entire wall was of mortared rubble, which set within wooden shutters. The Normans were already developing powerful engines for siege-warfare, and such towers could be adapted for castle building; though they also used high cranes (medieval crane-wheels can still be seen in the towers at Canterbury, and at Salisbury and Peterborough cathedrals).