The roofless building rear left looks like a classic “army fort” building -- log construction and second story overhang.

At first I did not think that this photo was that old since the guy in the middle was giving us a thumbs up. I didn't realized he was hiding this fingers. Maybe the first thumbs up ever.

Mathew Brady employed some 23 photographers in his project to photograph the Civil War. He did some photography himself (1st Bull Run, Fredericksburg ... ), but a large portion of the project was shot by others. The images were released by Brady's studio and as a result were very frequently credited to Brady himself rather than the photographer who actually shot the images. In recent years, many of the images that were originally credited to Brady have been re-attributed to the actual photographer.

While trying to google the current location, I came across a couple of sites that attribute the photo to A. J. Russell (formerly to Matthew Brady.) For example:

https://www.metmuseum.org/art/collection/search/268069

[Mathew. Not "Matthew." - Dave]

http://www.connectionnewspapers.com/news/2003/jan/29/when-union-mills-st...

The Orange & Alexandria RR never got anywhere near the Union Mills that Google shows us now.

This Union Mills was on Bull Run, close to Manassas Junction, which makes more sense given the apparent strategic importance evidenced by all the guards.

It does show up on some old railroad maps, such as this one from the Library of Congress:

http://www.loc.gov/item/91686259/

Still no joy in terms of any surviving remnants however.

This immediately brought to mind Robert Enrico's film of Bierce's "Occurrence at Owl Creek Bridge" that was broadcast on the original Twilight Zone.

Is there any way to pinpoint this location??

Entering "Union Mills, Virginia" into Google Maps brings you to a very, very rural area near Lake Monticello, and Charlottesville is more than just a few miles away. I guess nothing, not even a single piece, remains of these structures in the background???

This looks like the epitome of misery.

For the backup on the bridge behind. I almost missed it.

The poor fellows in back didn't get much exposure.

This would make a great album cover for a bluegrass group.

The officer holding the saber seems to have a mild case of the Bonaparte hand-in-jacket affectation.

There is one firearm, the pistol carried by the officer (who looks like a young Dustin Hoffman).

Looks like they're ready for a swordfight and hand-to-hand combat since there's nary a gun in sight!