Douglas County is the seventh-most populous of the 64 counties of the U.S. state of Colorado. As of the 2010 census, the population was 285,465. The county seat is Castle Rock.Douglas County is part of the Denver-Aurora-Lakewood, CO Metropolitan Statistical Area. It is located midway between Colorado's two largest cities, Denver and Colorado Springs. Douglas County has the highest median household income of any Colorado county or statistical equivalent.