Following the Norman Conquest of 1066, William the Conqueror established a massive network of over 30 "royal" castles to secure his grip on England. His most iconic possession was the Tower of London (specifically the White Tower), which served as his primary fortress and residence. Other major royal strongholds directly founded or significantly expanded by him include Windsor Castle, which remains the oldest occupied castle in the world, and Warwick Castle. His strategic "chain" of castles also featured Dover Castle (the "Key to England"), Hastings Castle, Pevensey Castle (where he first landed), and Colchester Castle, which was built on the ruins of a Roman temple. In the north, he established York Castle (Clifford's Tower) and Lincoln Castle. While many of these were initially timber "motte-and-bailey" structures, William's reign saw the transition to stone fortifications, marking the beginning of the great age of English castle-building that shaped the landscape of Britain for centuries.