Summary - a soaring rock summit visible from 4562 with awesome views of MSH, Castle Lake, Mount Adams and Mount Rainier. The summit road is 2WD friendly and the hike is steep but short. Bring a mast and don't count on two meters for contacts. ATT service is good, as is APRS. Combine with 4562 and Spud Mountain for triple the fun.
TL;DR - Castle Peak is a fifteen minute drive from 4562. (See my trip report on 4562 for general directions to this area.) The summit road begins at 46° 14.60', 122° 18.32', is rocky and rough but 2WD navigable, and ends in a broad flat area with plenty of parking. There is an OHV trail leading from here to right below the top.
The summit is a small, unshaded rock outcropping with sharp drops on two sides. As on 4562, the views alone are worth the drive. You will have a partial view of the breech on the north side of Mount Saint Helens and good views of Mount Adams and Mount Rainier. A mast will be handy for getting a wire up as the nearest trees are below the summit. ATT service is good, as is APRS. Don't count on two meters. One caution: this area is quite popular with hunters; it pays to know when they will be active. I was visited by a bow hunter scouting for elk while I was on top.
In summary, the hike is short, access is good, the views are awesome, and there are two more four point hills within easy striking distance. All in all, this is a very good SOTA destination.