William the Conqueror built the first Windsor Castle around 1070 as a wooden motte-and-bailey fortress guarding the River Thames a day's march from London. Over nine and a half centuries it was rebuilt in stone, expanded into a royal palace, and is still used as a working residence today — making it the oldest continuously occupied castle in the world and, at 13 acres, the largest occupied castle anywhere.
Windsor Castle has been home to 40 monarchs since William I. The Round Tower alone has been rebuilt or heightened at least three times — most dramatically by Jeffry Wyatville in the 1820s, who raised it roughly 9 metres purely to improve its silhouette from the Long Walk.
A 3D cutaway of Windsor Castle showing how William the Conqueror's 1070 timber motte-and-bailey grew into the world's oldest continuously occupied royal residence, with an era slider to step through its architectural history.
The Round Tower still sits on the original 11th-century motte; the curtain wall follows the same circuit Henry II rebuilt in stone after 1170; St George's Chapel and the Order of the Garter arrive under Edward III; and the Georgian State Apartments reflect Wyatville's 1820s remodel.
Drag the Era slider through 1070, 1170, 1350 and today to watch the wooden fort become a stone royal palace. Toggle the interior cutaway to fade walls and roofs and peer inside the Round Tower, chapel and staterooms.
Windsor Castle covers about 13 acres and has housed 40 monarchs since 1070, making it both the oldest and the largest occupied castle in the world.