Beginning at O’Leary Mountain Trail #3321 near O’Leary / Castle Rock Trailhead, Castle Rock Trail #3506 heads northeast along the base of Macduff Mountain. The trail passes through an old mixed conifer forest, crossing Cougar Creek and continuing northeast to Forest Road 480. The trail crosses the road and sidehills west up the southeast side of Castle Rock. The trail switches back, continuing a steep uphill grade to its terminus at a scenic viewpoint at the top of Castle Rock. — 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.