Castle Rock State Park is located a few miles from the "Saratoga Gap", which is the junction of Hwy 35 ("Skyline") and Hwy 9.  Road and mountain bikers in the Bay Area will be familiar with this area, and on weekends the parking lot where these highways meet is busy with cyclists starting their rides.  There are two parking lots for this park, the first a few miles past the Saratoga Gap on Hwy 35 south.  This is the main lot where the ranger station is located.  If you have reservations for the hike-in campground, you'll need to stop here first and get a parking pass.  The parking lot for the trail head is just a bit further down Hwy 35.  When I visited it was the offseason and we had to unlock the gate in order to get access to the lot (the ranger gave us the combination to the lock).  I'm not sure if this is the case in the spring/summer/fall.

The campground is about a 2.5mi hike in from the trail head at the parking area. Although there was a fire road at the campgrounds that looked like it maybe could be used to drive in and drop off gear, I didn't see anybody doing it, so be prepared to carry everything on your back.  In other words, don't expect to get a Yeti cooler full of beer out to the campsites.  

The hike itself is great, offering sweeping views of the Santa Cruz mountains and the Pacific ocean to the west.  There's even a waterfall right at the beginning of the hike.  There are a couple of different trails you can take, but the most scenic one IMO is the one that follows a west facing ridge.  There's a couple of spots where you'll need to do some scrambling up the rock faces, but nothing to worry about.  I wouldn't bring a toddler on it, but anybody older should be able to handle it.

The campsites themselves are typical California State Park "primitive" sites.  There are pit toilets, fire rings, and picnic tables.  Fires are increasingly banned in the dry months (i.e most of the year) in California now, but in January, when I was there they were allowed.  Because carrying firewood on the hike would have been a huge effort, the rangers kindly stocked bundles of wood at the main entrance to the campground with an iron ranger for making payment via the honor system. 

IMPORTANT NOTE: Although there is a communal water source at the campsite entrance, the water was deemed not safe to drink when I was there, and that had been the case for a while. The water tank apparently needs to be decontaminated.  So, we brought all our water in with us as well as water filters so we could get water from the nearby creeks.  Again, in winter, this isn't a problem. The way the drought has been going for the last decade in all of CA, I'd guess these creeks are running dry by mid May, if not sooner.  If you go in the summer, bring lots of water.

All in all, this is a great getaway for being so close to the megalopolis that is the San Francisco Bay Area.  As the crow flies, you're probably only about 25 miles from San Jose. But you'll feel like you're a lot further away.  And that's the whole point.

Why 4 stars instead of 5?  Please see this explanation of my ratings system.

5 stars - Worth traveling from far distances to visit.  Minimum of 3 night stay recommended in order to fully explore the area. A special place worthy of repeat visits. Example: Yosemite, Tahoe, Plumas

4 stars - Worth visiting if you're in the area as part of a larger trip.  For locals (i.e those within a few hours driving distance), a top tier spot, one that you'll visit regularly, maybe even annually. Example: Pinnacles, Lopez Lake, Lake Sonoma

3 stars - Convenient if you're a local. If coming from out of state, a convenient "home base" for nearby attractions, but not particularly beautiful or scenic itself. Example: Flying Flags RV Resort (Buellton), KOA Petaluma

Under 3 stars - Don't bother. It's an area that's not really worth exploring, or you can find something better in the same area.