Because the park sits between prairie and montane communities, a mixture of wildlife species representative of both ecotypes occurs at Castlewood. Mule deer, coyote, cottontail rabbit, porcupine, ground squirrel and red fox are commonly seen. Black bear, mountain lion and elk are not residents but occasionally migrate through the area.
A few rattlesnake dens may exist, and several rattlesnake sightings are reported each summer. Other reptile and amphibian species include western terrestrial garter snake, eastern fence lizard and Woodhouse’s toad. Cherry Creek supports the warm-water fish species of longnose dace, fathead minnow and black bullhead. The park contains the primary habitat types for Preble’s meadow jumping mouse, a federally listed, threatened species and the state-listed northern leopard frog.
Over 100 bird species nest and raise their young in the park. Turkey vultures roost along the canyon rim, riding thermal updrafts created as the rocks warm each morning.
For many years a unique dry-land heronry occurred in the park, where great blue herons nested in Douglas fir trees near a historic dam and reservoir site. However, the herons have not returned for the last three years.
Ground-nesting birds, such as western meadowlark, vesper sparrow and horned lark breed in the park. Several raptors may be observed as well, including golden eagle, Swainson’s hawk, Cooper’s hawk, red-tailed hawk and prairie falcon.