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is a Home Rule Municipality that is the county seat of Douglas County, Colorado, United States. Castle Rock is located about south of Denver and north of Colorado Springs on the Interstate 25 corridor just east of the front range of the Rocky Mountains. The town is the center of the burgeoning urbanization of the county. Castle Rock is named after a small prominent butte just north of the town, clearly visible from Interstate 25. Public access is provided to climb to the top of the rock. Castle Rock is on East Plum Creek, a small stream which periodically floods. As of 2005, the city is estimated to have a total population of 35,745. Castle Rock is now the 19th most populous municipality in the State of Colorado.
Castle Rock was founded in 1874 when the eastern Douglas County border was redrawn to its present location. Castle Rock was chosen as the county seat because of its central location.
The region in and around Castle Rock was originally home to Native Americans of the Arapahoe and Cheyenne tribes. They occupied the land between the Arkansas and South Platte Rivers. White settlers were drawn by rumors of gold and by land opened through the Homestead Act of 1862.
One of the first settlers in the area near today's Castle Rock was the original homesteader, Jeremiah Gould. He owned about to the south of "The (Castle) Rock." At that time, the settlement consisted of just a few buildings for prospectors, workers, and cowboys. In 1874 Jeremiah Gould donated to the new town that was also now home to the Douglas County government. For the beginning the six streets named Elbert, Jerry, Wilcox, Perry, Castle and Front were laid out to build the actual town of Castle Rock. The Courthouse Square was defined and about 77 lots, each 50 by , were auctioned off for a total profit of US$3,400.
It was not gold that put Castle Rock onto the map. The discovery of rhyolite stone made the reason to build a settlement that would become Castle Rock.
A new train depot brought the Denver and Rio Grande Railway to the area. The depot building now houses the Castle Rock Historical Museum on Elbert Street, where visitors can see history of how Castle Rock changed over the years. Castle Rock currently encompasses about , with a population of more than 42,000 in town and 70,000 in the surrounding area.
Until cancelled in 2007, The International, a PGA Tour tournament, was held every August in Castle Rock at the Castle Pines Golf Club.