In the front of the base, made mostly of stone, is this niche lined with bricks, leading to an inner stone wall.
This is the west side of the furnace, taken after walking slightly uphill. The path to the right leads further up the hill. Note how the upper brick section is very much off-center with respect to the middle circular section.
From further uphill, this is the southern face of approximately the upper half of the furnace. The adjacent road and Codorus Creek are in the lower background.
Here's just a bit of Codorus Creek, across the road from the furnace.
More about Codorus Furnace may be found at Waymarking and the official site of Hellam Township, Pennsylvania.