Castle Rock, CO has a population of 53,789 and is the 747th largest city in the United States. The population density is 1,574 per sq mi which is 3060% higher than the Colorado average and 1637% higher than the national average. The median age in Castle Rock is 35 which is approximately 5% lower than the Colorado average of 36. In Castle Rock, 68% of the population over 15 years of age are married, 94% speak English and 4% speak Spanish. 39% of Castle Rock residents were born in Colorado, 55% were born out of state, 2% were born outside of the United States and 5% were foreign born.