Castle Rock State Park is a 5,242-acre (2,121 ha) state park of California, United States, located along the crest of the Santa Cruz Mountains and almost entirely in Santa Cruz County, with parts extending into Santa Clara County and San Mateo County. It embraces coast redwood, Douglas fir, and madrone forest, most of which has been left in its wild, natural state. Steep canyons are sprinkled with unusual rock formations that are a popular rock climbing area. The park is named after a sandstone formation called Castle Rock. The forest here is lush and mossy, crisscrossed by 32 miles (51 km) of hiking trails. These trails are part of an even more extensive trail system that links the Santa Clara and San Lorenzo valleys with Castle Rock State Park, Big Basin Redwoods State Park, and the Pacific Coast. Due to its overnight parking lot, Castle Rock is a popular starting point for the Skyline-to-the-Sea Trail, a 30-mile (48 km) trail that begins near by at Saratoga Gap and leads to Waddell Beach north of Santa Cruz. There are two walk-in campgrounds within the park for overnight backpacking.

The Santa Cruz Mountains are a mountain range in central and Northern California, United States, constituting a part of the Pacific Coast Ranges. They form a ridge down the San Francisco Peninsula, south of San Francisco. They separate the Pacific Ocean from the San Francisco Bay and the Santa Clara Valley, and continue south to the Central Coast, bordering Monterey Bay and ending at the Salinas Valley. The range passes through the counties of San Mateo, Santa Clara, and Santa Cruz, with the Pajaro River forming the southern boundary.