This ultra-classic, super-short finger-crack is quite a popular line at castle and seldom feels very easy. There's a good rest at the handjam pods high on the route. Some consider it cheating to reach out to the rail at any point that is not the very top of the horn. Also, watch out on warm days as the crack and the super-polished footholds can get very greasy. Technically the route continues up to a rap anchor and several more pitches to the top of Castle Rock, but this is rarely climbed. There is a route description for this in the Leavenworth guide.
Location
Very obvious at the base of Upper Castle, just left of the intimidating Damnation Crack.
Protection
Several small cams. Typically the green, yellow, and grey aliens work great. Also bring two shoulder length slings to sling around the horn at the top of the crack (for the anchor). From the ground, you can pretty easily flip the slings off the anchor with the rope when you're done TRing the route.
For full value (imho) step left at the top of the finger crack to the obvious offwidth(short) that leads to a ledge with rap slings. If you wish to pro the offwidth bring a BIG cam ie #6 friend or likewise. However, there is a fixed pin at the begining of the offwidth, making it manageable without large a cam. When in doubt.....