The climb up is hard, but doable and a lot less of a hassle than the road, which only spits you out about 3 miles up the trail.
This is a semi-rocky, very well-draining trail through old growth forest with numerous challenging switchbacks and flowy corners. King-Castle will connect with USFS Trail #3506 - Castle Rock. It's worth riding that trail up to the summit.
Directions: From HWY 126 turn on to McKenzie Scenic Road, you'll enter the town of Rainbow, OR. And there will be a covered bridge you can drive across to your west. Take that, take a left after crossing and follow the signs to the trailhead. Park at the King-Castle trailhead. Ride up trail and enjoy.