Henry I died in 1135 and was buried in Reading Abbey. On his death the peace of the county was disturbed by civil war, for the crown was claimed by Henry's nephew, Stephen of Blois, though he had sworn to support the cause of Henry's daughter Maud or Matilda. Matilda had been married twice, and as her first husband was Henry V, the Emperor of the Holy Roman Empire, she is known as the Empress Matilda. War between Stephen and Matilda began in 1139 and spread over most of England. Windsor and Reading were held for Stephen, whilst Brian of Wallingford, a great magnate in Berkshire, took the side of Matilda. Wallingford Castle was besieged by Stephen in 1139 and again in 1145, but without success. A castle at Faringdon built by Robert Earl of Gloucester was taken and destroyed by Stephen. In 1145 Matilda gave up the contest and retired to France, but in 1152 her son Henry renewed the war and Stephen again besieged Wallingford and again unsuccessfully. He also besieged Newbury Castle, which was held by John Marshal of Hampstead Marshall. Eventually in 1153 peace was made at Wallingford--Stephen to be king for life and to be succeeded by Henry, son of Matilda. Stephen died in the next year, 1154, and Henry was crowned as King Henry II. He possessed himself of Wallingford Castle and held a Council there in 1155. Henry added to the buildings at Windsor Castle, and the lower part of the south side of the Upper Ward dates from his time. In 1163 a duel or wager of battle was fought between Robert de Montfort and Henry of Essex on an island in the Thames below Caversham Bridge. Essex was accused of treachery or cowardice, having thrown away the standard in a battle at Coleshill. He was defeated in the duel and was allowed to join the community of Reading Abbey.