 Originally designed to protect Norman dominance around the outskirts of London and oversee a strategically important part of the River Thames , Windsor Castle was built as a motte @-@ and @-@ bailey , with three wards surrounding a central mound . Gradually replaced with stone fortifications , the castle withstood a prolonged siege during the First Barons ' War at the start of the 13th century . Henry III built a luxurious royal palace within the castle during the middle of the century , and Edward III went further , rebuilding the palace to make an even grander set of buildings in what would become " the most expensive secular building project of the entire Middle Ages in England " . Edward 's core design lasted through the Tudor period , during which Henry VIII and Elizabeth I made increasing use of the castle as a royal court and centre for diplomatic entertainment . 
