A bridge castle (German: Brückenburg) is a type of castle that was built to provide military observation and security for a river crossing. In the narrower sense it refers to castles that are built directly on or next to a bridge. Sometimes, however, castles close to a bridge are referred to as bridge castles. These fortifications were often designed as toll castles that charged river tolls and were occupied only by a guard force. In Europe several examples of bridge castles have survived, especially in the south and southeast of the continent.