This trail system is mostly single track, and varies from long flat stetches on the Meremac River bottoms, to some short hill climbs and descents as you get away from the river. If you start your trek on the bottoms, you can head westward along the river until you find a railroad bridge which you can ride under (Cedar Bluff loop/Blue Ribbon Loop as some like to call it). This will take you to a loop trail that is quite fast and fun up in the hills. When you return on the bottoms, you can then hit the trails around the parking lots (there are several). Most are well marked and well ridden singletrack. Nothing especially challenging, but all quite enjoyable for an afternoon of riding. Castlewood State Park has a website with a good map of the trails. This is a hiker/biker shared system and it can be quite crowded. 15.5 total miles of trail in the park.
Castlewood State Park (from their website). From I-270 take the Dougherty Ferry Road exit and travel west on Dougherty Ferry to Big Bend Road. Turn right onto Big Bend Road and travel west to Ries Road. Turn left onto Ries Road and continue until reaching the park entrance at the intersection of Kiefer Creek and Ries roads.
http://www.singletracks.com/item.php?c=1&i=1108