A parapet is a low protective wall along the edge of a roof, balcony, bridge, or castle wall. Think of it as a safety barrier that keeps people from falling off high places while also letting them see over the edge.

On medieval castles, the parapet was the chest-high wall at the top where defenders could crouch behind for protection while shooting arrows at attackers below. Those rectangular notches you see cut into castle walls are called crenellations, and they're part of the parapet, giving archers spots to shoot through while staying mostly protected.

Today, you'll find parapets on the edges of apartment building roofs, along highway bridges, and atop parking garages. They serve the same basic purpose they always have: keeping people safe from dangerous falls. Modern parapets might be made of concrete, metal, or brick instead of stone, but the idea remains unchanged.

When you're standing behind a parapet, you're protected but not hidden, secure but still able to see what's happening on the other side.