What did I do?
The picture itself is very low quality, so most of the work was working with pixels. First I cleaned up the sky, getting rid of specks and smudges, and generally whitening the whole area. Next I repaired the holes on the ground and on the people, re-imagining lost information. Then I filled in the edges, re-imagining lost details along the right edge. Finally, after removing the noise from the ground and cannon (a rather difficult task given the pixelated nature of the photo), I darkened the shadows as well as the faded faces of the men, bringing more detail into focus.
*Original photo taken with gratitude from the Library of Congress, Prints and Photographs Division. Photographer unknown, circa 1910.