That second photo has a pentagonal building under the south end of the bridge. It's an old
fort from the Civil War era that became instantly obsolete with the invention of the rifled
cannon. The outside corners are at 60 degree angles. I got a tour one day with my boss
and some co-workers when we were out "team building". The National Park Service operates
the fort as a museum. We saw the powder room (gunpowder -- not women's cosmetics
and learned a lot of interesting things.
Lesson learned: Take some time to look around. You can learn some amazing stuff.