Valnerina was a location in the Italian state of Umbria, which was formed by the river Nera in the mountains and was known for its winding roads and steep narrow passes. Naturally, it was a particularly challenging landscape to navigate.
During the turn of the 16th century, the valley was used by Cesare Borgia to produce a large number of bombers designed by Leonardo da Vinci. These machines were each equipped with a single cannon and were intended for use in Cesare's conquest of Italy.
However, the production facilities were destroyed by the Assassin Ezio Auditore da Firenze, who stole one of the bombers and used its weaponry to dispose of the factories, allowing no more to be created, before crash landing the invention into a sentry tower and destroying it permanently.