A curtain wall is an external cover of a building or a structure which has non-structural exterior walls. These are made to protect the walls from weather change. It is made of a lightweight material to reduce construction cost. Sometimes even glass may be used as a curtain wall so that abundant natural light can penetrate to the building. In castles, the fortified walls surrounding the walls consist of curtain wall. In modern architecture, the curtain walls comprise large panes of glass.