Located in central Colorado at the junction of Interstate 25 and State Highway 86, Castle Rock is 28 mi south of downtown Denver and 37 mi north of Colorado Springs.
The town lies a few miles east of the Rampart Range of the Rocky Mountains on the western edge of the Great Plains. Castle Rock, the butte that is the town’s namesake, sits just north of the town center. Other prominent landforms visible from Castle Rock include Dawson Butte, Devils Head, Mount Evans and Pikes Peak.
East Plum Creek, a stream within the South Platte River watershed, flows generally north through Castle Rock. Hangman’s Gulch, which runs northwest then west around the north side of the town center, drains into East Plum Creek as do multiple unnamed gulches in the southern and western areas of town. McMurdo Gulch and Mitchell Gulch run north then northeast through eastern Castle Rock and drain into Cherry Creek east of town.
Castle Rock is within the Colorado Foothills Life Zone. The hillsides are covered with large meadows of grass, small plants, scattered juniper trees and open ponderosa pine woodlands. Other trees common in the area include Gambel oak (scrub oak or oak brush) and pinyon pine. Local wildlife includes the American badger, American black bear, bobcat, coyote, Colorado chipmunk, gray fox, mountain cottontail rabbit, mountain lion, mule deer, pocket gopher, porcupine, and skunk. Birds that can be found in the area include the golden eagle, peregrine falcon, sharp-shinned hawk, black-billed magpie, red-tailed hawk, pinyon jay and western tanager.
Lying within the Front Range Urban Corridor, the town is part of the greater Denver metropolitan area. Castle Rock borders three communities, all to its north; from west to east, these are Castle Pines Village, the city of Castle Pines, and The Pinery. Other nearby communities include Franktown to the east, Larkspur to the south, Perry Park to the southwest, and Sedalia to the northwest.