After the Battle of Hastings in 1066 William led his army to Canterbury and then attacked towns along the Pilgrims' Way, including Guildford. Later William, or one of his barons, built Guildford Castle, There is no record of it in the Domesday Book so construction probably started after 1086

The Shell Keep Guildford Castle
Late 11th or early 12th century, a wall made of Bargate stone was built around the top of the motte creating what is known as a shell keep, and then around the 1130s a keep (tower) was added, again made of Bargate stone from nearby Godalming bonded with hard and durable mortar.[3][5] The keep may have been built over part of the shell keep an its foundations went down to the chalk bedrock.[3] The general form was quadrangular, its exterior dimensions being 47 feet (14 m) by 45.5 feet (13.9 m). The walls are about 10 feet (3.0 m) thick at the base [2] and taper towards the top.