During the Holy Roman Empire, Germany's Rhine River was a major trade artery (and provided tax revenue). The numerous castles along the Rhine were toll stations, and most merchants had to travel sizable lengths of the river to arrive at their destination. Although the emperor's closely guarded the right to charge a toll, local nobility would occasionally break the law and build a castle along the Rhine to charge a toll (hence the origin of the phrase "robber baron"). When possible, such "robber castles" were destroyed to prevent the tragedy of the commons. What is the commons resource here?