| | Castlewood, SD | Bryant, SD | United States |
| Population | 680 | 721 | 329,725,481 |
| Female Population | 49.0% | 45.5% | 50.5% |
| Male Population | 51.0% | 54.5% | 49.5% |
| Median Age | 34 | 23.4 | 38.4 |
| space3 | | | |
| Population - 2010 | 628 | 456 | 309,327,143 |
| Population - 2000 | 666 | 396 | 285,036,114 |
| Population - 1990 | 549 | 374 | 251,960,433 |
| Pop. 2000 to Now | -2.7% | 15.4% | 15.6% |
| Pop. 1990 to Now | 18.0% | 22.2% | 30.8% |
| space4 | | | |
| Population Density | 591.3 | 1,376.0 | 93.4 |
| Land Area | 1.2 | 0.5 | 3,531,905.4 |
| Water Area | 0.0 | 0.0 | 264,836.8 |
| space | | | |
| Est. Total Population By Age | | | |
| | Castlewood, SD | Bryant, SD | United States |
| Age 0 to 4 | 7.6% | 16.8% | 5.9% |
| Age 5 to 9 | 8.5% | 9.7% | 6.1% |
| Age 10 to 14 | 8.1% | 8.2% | 6.6% |
| Age 15 to 17 | 5.7% | 3.1% | 3.9% |
| Age 18 to 20 | 5.4% | 4.4% | 4.0% |
| Age 21 to 24 | 4.3% | 9.8% | 5.2% |
| Age 25 to 34 | 12.6% | 10.5% | 13.8% |
| Age 35 to 44 | 10.4% | 10.5% | 12.9% |
| Age 45 to 54 | 12.4% | 2.5% | 12.6% |
| Age 55 to 59 | 5.1% | 2.2% | 6.7% |
| Age 60 to 64 | 5.9% | 3.6% | 6.3% |
| Age 65 to 74 | 7.2% | 7.2% | 9.6% |
| Age 75 to 84 | 3.2% | 4.0% | 4.5% |
| Age 85 and over | 3.4% | 7.4% | 1.9% |
| space1 | | | |
| Race | | | |
| | Castlewood, SD | Bryant, SD | United States |
| White | 93.7% | 89.2% | 59.4% |
| Black | 0.0% | 0.4% | 12.2% |
| Asian | 0.0% | 0.0% | 5.6% |
| Native American | 0.0% | 0.0% | 0.6% |
| Hawaiian, Pacific Islander | 0.0% | 0.0% | 0.2% |
| Other | 1.2% | 0.0% | 0.4% |
| Two or More Races | 4.1% | 0.1% | 3.2% |
| Hispanic | 1.0% | 10.3% | 18.4% |
| space2 | | | |
| Marriage & Family | | | |
| | Castlewood, SD | Bryant, SD | United States |
| Married Population | 55.5% | 54.8% | 49.8% |
| Currently Married | 55.5% | 54.8% | 48.0% |
| Married but Separated | 0.0% | 0.0% | 1.8% |
| Single Population | 44.5% | 45.2% | 50.2% |
| Never Married | 29.5% | 25.1% | 33.8% |
| Divorced | 11.7% | 10.0% | 10.8% |
| Widowed | 3.3% | 10.2% | 5.6% |
| space5 | | | |
| Household Size | 2.75 | 3.15 | 2.60 |
| Households | 247 | 207 | 124,010,992 |
| Family Households | 171 | 144 | 80,755,759 |
| Married couple, w/children | 38.0% | 48.6% | 30.5% |
| Married couple, no children | 41.5% | 32.6% | 42.9% |
| Non Family Households | 76 | 63 | 43,255,233 |
| Single householder, w/children | 11.1% | 6.3% | 15.8% |
| Single householder, no children | 9.4% | 12.5% | 10.8% |