I've visited the site several times and taken some pictures, but not with my most recent (and only digital) camera. The open area in front of the furnace includes several trees, whose presence in my pictures is pretty much inevitable. In this shot, the section in which the iron was produced, which is shaped like a truncated pyramid, is behind an open section having a wooden roof.
Further back are some stone walls. U.S. 15 is somewhere behind then.
On a small hill just north of the furnace are the ruins of the iron master's house.
Here are the ruins from a different angle.
A trail connects the furnace and iron master's house to an entrance of Cunningham Falls State Park. Along the way is this bridge over Little Hunting Creek.
The trail also includes a bridge over U.S. 15, which you see here, looking northward.
The bridge was appropriately decorated for Memorial Day.
I retraced my steps back to the furnace and took another picture of it, and a tree trunk.
To learn more, go to Catoctin Furnace Historical Society, Maryland Historic District and Atlas Obscura.