Lincoln Castle is an 11th century earthwork motte and bailey fortress, founded by William the Conqueror. Built on the site of the original Roman Fort of Lindum, the 12th century curtain wall is flanked by two mottes, two gates, and Cobb Hall, a large round tower. On one motte stands the square 14th century Observatory Tower, while a 12th century shell keep crowns the other motte. The East Gate is flanked by two 13th century round turrets and was protected by a barbican, the massive square West Gatehouse and barbican was sealed in the 14th century and only re-opened in 1993. A Georgian building in the bailey contains one of the original Magna Carta documents sealed by King John. To the west of the castle is the Bishops Palace. 9 miles south is Somerton Castle and 10 miles north-west is Torksey Castle.