A curtain wall is a common feature of medieval castles. It is a set of walls that surround and protect the interior (bailey) of the castle. Walls were often connected by a series of towers or mural towers to add strength and provide for better defense of the ground outside the castle. Additional provisions and buildings were often enclosed by such a construction, designed to help a garrison last longer during a siege by enemy forces.
"Curtain walls" have evolved to take a prominent place in modern architecture. Like the ancient walls mentioned above they are for protection, but from the elements, not maurading hordes. Unlike the castle walls, modern curtain walls are not structural, they are an attachment. The curtain wall framing, usually aluminum, is "hung" on the structural steel by bolting or welding, and the glass and between floors spandrel is installed in the framing. Massive modern "castles of commerce" like the Sears Tower in Chicago are curtainwalled.
