Beginning at King’s Castle Trailhead, King Castle Trail #4326 travels south and east up the northern and eastern side of Castle Rock. The trail meanders uphill over several streams and through an old growth coniferous rainforest. The trail switches back in some steep sections, climbing to and crossing Forest Road 480 before heading west and ending at Castle Rock Trail #3506. — Description from the US Forest Service website.
Most people ride this trail as part of a clockwise loop with Olallie, O’Leary, Castle Rock and King Castle. That way you can climb a significant amount of road which will get you pretty high up on the loop before decending 100% singletrack. The full loop is over 30 miles and 6000+ ft elevation — so brace for a long day.