Why were the castles built along the Rhine River?

1. easy access to water for the residents

2. in medieval times land transport was inefficient - the roads were usually poor, unpaved and carts could carry small quantities of goods. being located next to a river meant supplies could be transported by ships/boats - they carry more cargo more effectively.

3. protection - if one side of the castle was built along the bank of the river, approaching the castle from this direction would be difficult. The enemy must cross the river slowly in boats, while coming under fire from the soldiers in the castle.