The Castle Crags state park has a couple of parking lots and a ranger station. The first parking lot is right off the Castle Crags State Park exit you take on Interstate 5, the second parking lot is accessed by taking a right after the ranger station and following the road for another half mile as it snakes its way up a narrow one lane road (which supports two lanes of traffic!) to the second parking lot.
Regardless of which parking lot you park in, I recommend getting a map at the ranger station as the trail is well signed but can also get confusing.
You'll take the trails marked "Crags Trail" initially through a heavily forested area as the moderately steep hiking starts, on a few occassions you'll get really nice views of the crags jutting up into the sky.
Near the end of the wooded area, you'll see a sign for Indian Springs, a popular spot for hikers and those having a picnic or relaxing. You'll want to keep taking the trail, however, and it starts getting steeper and rockier.
The steep trail keeps going until you reach the top and find yourself on a fairly large summit plateau with the crags around you, including Castle Dome. From this hiking trail approach, the rocks of the domes and outcroppings here are fairly smooth and steep, so I'd rate them at Class 3/4, so some fun can be had going even higher on these rocks if you have some appropriate climbing equipment (shoes, rope, etc.).
The weather is something to watch out for here. I've done this hike in the end of January and it was easy to get up with just some patches of snow and ice and incredible views of Mt. Shasta. But, I also attempted to get up here in late March and a recent storm dumped about 4 feet of snow on the trail and it repulsed about 4 of us trying to get up without any equipment that would have helped (snowshoes, poles/ice axe, rope, etc.).
In total, you're hiking about 2,500' in 6 miles and it's probably around a 4 hour roud-trip with a good pace and small stay at the top where the trails ends near Castle Dome. In the 3 times I've tried this route, I've only seen a total of maybe 6 people in late Winter and Spring, don't know about Summer.
This is mostly a hiking trail during the non-Winter season with usual strategy of layers in cool weather. In Winter, you should consider snow shoes, ski/trekking poles or ice axe. If you want to do some further climing on the rock at the very top, then some class 3/4 or bouldering equipment would be helpful.
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