This is the route that I took from the West fork of the Rock Creek. The route is the same if your coming from Lake Fork over Sundance Pass.
WEST FORK of the ROCK CREEK TRAILHEAD
From the south side of Red Lodge Montana, turn West off of highway 212 and follow the road heading WSW across the plain and into the Rock Creek Valley. Follow the valley for 15-16 miles to the end of the road. This is the Trailhead leading up the West Fork of Rock Creek. The Rock Creek valley is popular for fishing, and several good camping areas can be found along the road leading from Red Lodge to the trailhead. From the trailhead, follow the path leading along the North side of the West Fork. After 7.5 miles you will reach tree line and a established bridge across to the South side of Rock Creek. Once you reach the bridge there are several good flat grassy areas to set up base camp. Such areas include Sundance Lake, Shadow Lake, or the Silt Lakes.
The route starts from our base camp at Shadow Lake. From the lake follow the West Fork of the Rock Creek up towards the Silt Lakes. Just past the Silt Lakes start climbing up towards Omega Lake. Once at Omega Lake strap on your crampons and climb to the top of the Omega Saddle. You can avoid the Omega Snowfield by hiking on the rocks to the right of the snowfield, but it's much faster to stay on the snowfield. Once at the top of the Omega Saddle you can take your crampons off and drop down into the bowl of Castle Mountain. I would drop immediately into the bowl and then again put on my crampons again. From here hike northwest to the top of the bowl. The hike isn't too physically demanding. Once at the top of the bowl you can head over to the Castle Mountain summit first if you want before heading over to Castle Rock Spire. The detour only takes an extra 20-30 minutes. Head over to the most northwest corner of Castle Mountain and then drop down into a saddle between Castle Mountain and Castle Rock Spire. You have now reached your destination of Castle Rock Spire. The actual climb of Castle Rock Spire can be found in a different route description.
It is almost essential at any time of year to have crampons and an ice axe to reach Castle Rock Spire. During late August and September it can be reached with out that gear but would require much more time to go around the snowfields.