Newark Castle is a well-preserved castle sited on the south shore of the estuary of the River Clyde in Port Glasgow, Inverclyde, Scotland, where the firth gradually narrows from the Firth of Clyde and navigation upriver is made difficult by shifting sandbanks. For centuries this location was used to offload seagoing ships, and led to the growth of Port Glasgow close to the castle on either side and to the south. When dredging techniques made the Clyde navigable as far as Glasgow the port became a shipbuilding centre, and the castle was surrounded by shipyards.