London was both the centre of English government and a hub of international trade, based on the tidal River Thames. Windsor Castle, originally built by William the Conqueror, was strategically located near the River Thames and Windsor Great Park, a royal hunting ground. It was extensively replenished and remodelled by Henry II in the 12th century. The eastern approaches of the Thames were flanked by the deep water docks at Tilbury and the thriving market town of Gravesend on the south bank, which was linked to London in the 15th century by the ‘Long Ferry’; river transport was much safer than the roads. The Essex marshlands raised sheep; the Kentish borders raised rebellion, Wat Tyler (1381) and Jack Cade (1450) both marched on London from Kent. Surrey turned Essex wool into cloth, using rare fuller’s earth mined at Reigate: gilforte cloth was named after Guildford, the centre of manufacturing. Waltham Abbey was a place of pilgrimage in the Medieval period and was frequently visited by monarchs on their hunting trips to Epping Forest.