Castle Rock State Park is located three miles south of Oregon, Illinois on Highway 2. The park is centrally located in the Rock River Hills region of Illinois, and its rolling topography is drained by the Rock River. The park is located along the west bank of the Rock River in Ogle County. The park is very representative of the Rock River Hills area with rock formations, ravines, and unique northern plant associations. A sandstone bluff, adjacent to the river, has given the park its name.

The basis for the acquisition of Castle Rock State Park is the protection of natural resouces that are unique to Illinois. A thin layer of glacial till covers this region and several distinctive plant species, remnants of the native forest and prairie, still exist. In one valley, 27 different types of ferns have been identified. The park is one of the largest significant natural areas in the northern part of Illinois.

Most of the outstanding natural features are located in the dedicated Nature Preserve which is protected by state law. Use of the 710-acre preserve will be restricted to scientific study and limited interpretation activities.

Illinois tribes of native Americans inhabited the region until 1730 when the Sauk and the Fox tribes, who were being forced westward by the colonists, invaded and inhabited the region. The native Americans called the river in the new territory "Sinnissippi," meaning "Rocky waters."

In the early 1800s the native Americans were again forced westward into Iowa. In 1831, Blackhawk, the Sauk chief, led his people in a series of raids back into Illinois to reclaim their ancestral lands. The battles that followed were known as the Blackhawk Indian wars. Chief Blackhawk was captured in 1832 and ordered to a reservation.

The Castle Rock area was settled by New Englanders early in the 19th century. This area was proposed as a state park in 1921 by the "Friends of Our Native Landscape," and they were able to acquire some of the land they described as "a unique wilderness remnant of great natural beauty and scientific interest." In 1964, the Natural Lands Institute, a non-profit natural lands preservation group, conducted a public fundraising campaign to preserve part of the Castle Rock area.

Castle Rock was recognized in the state of Illinois as an outstanding area of major scientific importance in 1965. It was established as a project area, and land acquisition by the State was started in 1970.

At the present time, Castle Rock consists of approximately 2,000 acres, 710 of which are designated as an Illinois Nature Preserve. The site was dedicated as a state park in 1978.

Castle Rock does not have a campground except for a primitive canoe camping area. Access to the area is by canoe or boat only.