Running east from White Sulphur Springs, the Castle Mountains are not the jagged gems so many climbers and hikers seek in Montana; in fact they are rather low and unremarkable by Montana standards.
However, the Castles, where hunters are probably the largest user group, offer some nice opportunities for rock climbing and solitary hiking.
The western side is wetter and is characterized by granite outcrops that give the range its name, and the drier eastern side features a lot of limestone.
Elk Peak, 8589', is the range highpoint, and I have attached the SP page for it here. As for rock climbing, there are three principal areas: Grasshopper Rocks, Limestone Ridge, and Green Canyon. Grasshopper and Green will soon appear as attachments to this page.
If you can, grab a copy of The Big Empty by Ron Brunckhorst. He and his brother Gerald have done most of the route development here. The Mountain Project page has very limited information right now, but it was my only source and is better than nothing.
Most of the range is accessible from various trunk roads off U.S. 12 east of White Sulphur Springs. See attached pages for specific details.
None in particular. Respect postings about private land and about area-specific locations.
There are Forest Service campgrounds, and there are a lot of options for dispersed camping.