You can read the backstory on the site here.
There are two places to park. The first is directly under the railroad bridge (39 15 03.42 N / 76 45 51.02 W) but the signs say you are subject to towing. The safer parking lot, with limited capacity, is a long walk away at one of the trail heads on the other side of the river (39 15 10.02 N / 76 45 36.87 W).
Either way you will go through the gate on the right side under the railroad bridge and, when you come to the level of the railroad track, turn left and take a short walk to old stairs at approx (39 15 03.35 N / 76 45 52.96 W)
At the top of the stairs you will enter a series of steep trails including some short stairs. When you get to what seems to be the top of the climb, bear left and proceed to the actual alter (39 15 02.46 N / 76 46 09.08)
You can shoot the remnants themselves but this place is calling out for a model shoot about death and fear.
There are some other relics such as an abandoned pool and water tower deeper into the woods, but the alter is 90% of the interesting stuff.
