Often overlooked, because it sits in the middle of many of its "bigger" brothers, Castle Peak is situated on the main Lost River Range divide between Lost River Peak and USGS Peak.

Getting There

Travel west of Arco along Highway 93, until you reach Mackay Reservoir. Turn North off the highway at mile marker 115 onto the dirt road which is signed "Upper Cedar Creek" (This is the same approach to Lost River Peak and the "Super Gully" route).
Pass through 3 barbed wire gates (please close them behind you) and follow the road to the right at the base of the mountains (Left fork will take you to the Super Gully start). Drive down the right fork spur road for ~1/2 mile and park at the end of the road.
A high clearance vehicle is not needed, but recommended.

Camping

Primitive camping at the end of the road. There is usually always water in Cedar Creek.
If you want a more civilized campground, there are trailer hookups, toilets and camping at Mackay Reservoir.

Getting to the pass

From the end of the road, follow an old but distinct trail up Upper Cedar Creek.
The trail begins on the West side of the creek, and you will cross the creek several times, but mostly stays on the West side. Look for the old blaze marks on the trees if you lose the trail.
After about 1-1 1/2 miles (Approx. Elevation 7500 ft)the trail becomes washed out/faint and crosses to the East side of the creek. If you find yourself doing substantial bushwhacking, stay close to the creekbed and start looking for the trail on the East side of the creek.(Old scarred blaze marks on the trees mark the entire length of the trail)
Once you find the trail again it will lead you into the upper canyon quite easily.
When you get to the last major fork in the creek stay to the right (East) fork and then it is cross country travelling from there to the pass at approximately 10,000ft elev.