Castel del Rio - Bridges Ponte a Schiena d' Asino The Donkey Back Bridge of Castel del Rio was built around the fifteenth century on commission by Obitzo Alidosi to master Andrea Gurrieri. The Bridge symbolized the power and solidity of the Alidosi family and presented itself with a structure called “donkey back” with its 42 meters arch. Inside, five rooms were built used by the guards for the collection of gabelles.