In castles, the surrounding fortified walls. In modern architecture, an outer non-load-bearing wall, often simply a field of large panes of glass held in place with a lattice of other material, sometimes merely thin metal bands. The modern curtain wall was first made possible with the introduction of the structural steel skeleton by watchmaker and inventor James Bogardus, using modular prefabricated cast iron and glass in New York City in 1849. Other notable progress in this direction was made by architects Louis Sullivan (American, 1856-1924), in the Carson-Pirie-Scott store (Chicago, 1899-1904), and Walter Gropius (German, 1883-1969), whose design for the Bauhaus (Germany, 1926) became the precursor to the "glass box" building of the International style.Examples: Ludwig Mies van der Rohe (German, 1886-1969), Friedrichstrasse Skyscraper, Berlin-Mitte, Germany, a project in 1921, this is a perspective drawing from the north, charcoal and pencil on tracing paper mounted on board, 68 1/4 x 48 inches (173.4 x 121.9 cm), Museum of Modern Art, NY. This design for a twenty-storey tower was based on the then-untried curtain wall idea: that a supporting steel skeleton would be able to free the exterior walls from their load-bearing function, allowing a building to have a surface that is more translucent than solid.