Yeah parapet has all these medieval connotations, but it's a word I use every day. You'll encounter the word in civil engineering, when dealing with box culverts.

The parapet is a flat piece of concrete on either side of the structure. It is perpendicular to the ground, and immediately below the structure's deck. The box walls intersect the parapet at right angles.

It's a little hard to picture, but if you take any old box and turn in on the side then stick a flat piece straight up from the box on either end, that's what the parapet looks like.

In box culverts, the parapet usually cracks where the box walls meet the parapet. This is not a Good Thing.