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http://www.castlewales.com/jf_gilbt.html John fitz Gilbert was the father of William Marshal Earl of Pembroke. John was the son of Gilbert, the marshal of the royal household of King Henry I. The office of the marshal was part of the Curia, with a deputy in the Exchequer and one in the King's Bench, as well as one in the Court of the Marshalsea of the King's household. The office was subordinate to the office of constable of the royal household.. The office was responsible for everything connected to the horses of the royal household, the hawks and the hounds as well. He had the general duty of keeping order in the royal court/household, arranging for the billeting of members of the court, keeping tallies and other vouchers of the expenditures of the household, keeping rolls of all who performed their military service there, and being responsible for the imprisonment of debtors. The
"Constitutio Domus Regis" gives the duties of the master marshal for King Henry I.

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http://www.castlewales.com/jf_gilbt.html
The rising and falling fortunes of neither side in this civil war greatly effected fitz Gilbert’s prosperity; he used his position and his castles in Wiltshire to continue to attack the lands of Stephen’s supporters. One of his frequent victims was Patrick constable of Salisbury, who was King Stephen’s man. After several years of this warfare, both men had had enough of the deprivations resulting from their attacks on each other. They worked out a compromise in 1141; John fitz Gilbert would put aside his first wife and marry Patrick’s sister Sibile [Sibyl], and Patrick came over to the Empress’s side. This compromise gave Patrick peace and relief as well as the later title and lands of the earldom of Salisbury. John nullified his most dangerous enemy and definitely increased his own social position by marrying into one of the great feudal families of England. It hurt neither man that they could both now raid the lands of Stephen’s supporters in Wiltshire, Berkshire and Hampshire.