Sussex, former county, SE England, since 1888 divided for administrative purposes into East Sussex and West Sussex. Lewes is the county seat of East Sussex; Chichester of West Sussex. The South Downs—low, rolling hills that cross Sussex from east to west and terminate at Beachy Head on the English Channel coast—are the former county's most notable geographical feature. The old kingdom of the South Saxons (see Sussex, kingdom of) was founded by King Ælle in the late 5th cent. Later the region was incorporated into Wessex. William I (William the Conqueror) landed at Pevensey in 1066 and defeated Harold's Saxons at Hastings. In 1974, East Sussex and West Sussex were reorganized as nonmetropolitan counties.