Today, we reinvent defensive fortifications. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them. The great American forts of the 19th century took a form most of us have forgotten. The towers and loopholed overhangs on the old Medieval castles could not stand up to cannon fire. Most 19th-century forts were square or pentagonal, with bastions on each corner.
Bastions are spade-shaped widenings of the corners.
They let defenders fire parallel to the walls. Bastions gave those old forts the shape of great stone snowflakes. Fort Sumter, out in Charleston Bay, was five-sided, but it sat on the tip of an island. Water came right up to four of its walls. Only the fifth wall needed the protection of bastions.