Castle Rock is a home rule town that is the county seat and the most populous municipality of Douglas County, Colorado, United States. The town population was 73,158 at the 2020 United States Census, a 51.68% increase since the 2010 United States Census. Castle Rock is the most populous Colorado town (rather than city) and the 16th most populous Colorado municipality. Castle Rock is a part of the Denver-Aurora-Lakewood, CO Metropolitan Statistical Area and the Front Range Urban Corridor. The town is named for the prominent, castle-shaped butte near the center of town.