Soldiers actually took advantage of the karst landscape, utilizing the sinkholes as well as the high ground of the hill.
If you follow the lower trail, you'll see the entrance to a cave. Signs warn "No trespassing" and tell that wild caves can serve as snake dens.
The Shenandoah Valley is known for it's karst topography and caves. Crystal Caverns on Hupp's Hill is a commercial cave but is temporarily closed.
Karst: A terrain, generally underlain by limestone or dolomite, in which the topography is chiefly formed by the dissolving of rock and which may be characterized by sinkholes, sinking streams, closed depressions, subterranean drainage, and caves.