This is from the web site posted below:
Balcony Rock is a "hidden" gem of an area right outside the community of Sandy Hook, just upriver from Maryland Heights. It has a rather expansive cliff that tops out over 80' tall at its highest point. There is a good bit of untapped routeage here for one to explore. People have been coming here for some time, as evidenced by the chalk and bolts on the main cliff. Parking is a problem, however, for this place: mainly, there almost isn't any. The nearest place to park is a turnaround pull-off a 100 yards or so away, immediately outside of the Sandy Hook community. This is the school bus turnaround, and there is a No Parking sign there. During the weekdays, don't even think about using this for parking - you will be ticketed and/or towed! During the weekends, as long as you keep yourself well off the road so people can still turn around (the locals use this as their primary turnaround spot, too), you should be okay. If you block them from being able to use it, though, you could find yourself with a nasty surprise at the end of your climbing day. This south-facing rock is an ideal place to play during the chillier months of the year (provided it is sunny out).
It looked like most of the climbs were easily top ropable. When facing the crag, go left up a small hill to the top of the rocks. There are numerous trees, cracks, boulders and features to set up a top rope.
Unlike Harper's Ferry, I don't think you need to register with the park service to climb this crag.