Ther are a multitiude of riding opportunities here. From aggressive single track to easy flat dirt roads. My favorite ride is the two loop trails through the roadless area. The official elevation gain is in the neighborhood of 2500 feet, however, with all the little drops and climbs, the true elevation gain probably exceeds 4000 feet. It is killer. This ride is for advanced riders. Another great single track ride is Yankee Jim Ridge. And for those that like to climb and then wail down the backside of some jeep roads, the road from Castletown to Grasshopper Campground is a work out. To top it all off, there is a Hot Springs resort, really just a pool and a hot soaking tub, in White Sulfur. So, no matter how rigorous the riding, the end of the trail is always soothing. And if the Castles aren't enough, the USFS has put together a trail system with an official mtn biking map, for riding opportunities in the Little Belt Mountains, just north of White Sulfur Springs. This map is available through the Lewis & Clark Nat'l Forest.
How to get there:
White Sufur Springs is in the center of the state. The Castle Mountains lie to the southeast and the Little Belts lie to the northeast of White Sulfur Springs. There are tons of camping opporunities in the nearby forests as well as a few motels in town.