 Castle Rock is part of the Longview, Washington Metropolitan Statistical Area; the population was 1,982 as of the 2010 census.Castle Rock is named for a volcanic rock outcropping over the Cowlitz River, "The Rock", rising 190 feet high on the south side of the city.The rock formation, resembling a castle, became a geographic landmark for Cowlitz Indians and Hudson's Bay Company traders as early as 1832.