Bridge_castle
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.[dubious – discuss]
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.
The bridge castle type—which is only rarely mentioned in detail in the specialist literature—is not always clearly distinguishable from the "fortified bridge". In medieval Europe, numerous river crossings were protected by tower structures and outworks.