Castle Rock sits at the base of Mount Diablo. It borders the Shell Ridge Open Space which is part of the Walnut Creek Open Space. Shell ridge includes the Old Borges Ranch. Frank Borges purchased an existing shack in 1899 along with 700 acres of land for his family home and cattle farm. The Borges 700 acres almost certainly included what is now Castle Rock. Cattle still graze in Castle Rock as well as the surrounding open spaces and parks.
Castle Rock gains its name from the beautiful windswept sandstone rock formations that can be found on the eastern ridge of the main canyon in the park. These rock formations and the caves that are carved out of them by the wind sweeping through the canyon have been a destination for hikers and rock climbers of the East Bay area for decades.
In addition to the trails and rock formations Castle Rock also features a large outdoor entertainment area near its main entrance. This area includes a pool, sandy volley-ball court, basket-ball hoops, soft-ball field picnic tables and barbeques. This area is often used by schools, businesses, families and groups of friends. Often times visitors are unaware of the expansive trail system that lies a short hike past this more heavily used section of the park.