Fords of Isen
The river Isen (Sindarin Angren) was not crossable anywhere south of its origin in Isengard, except at the Fords of Isen which were just above the point where the Isen bent west towards Belegaer. As it was the only way into Gondor and later Rohan, it was closely guarded from the fortresses of Isengard and the Hornburg at Helm's Deep.
During the War of the Ring the fords were attacked from both shores by Orcs of Saruman who had passed south from Isengard in secret, and King Théoden of Rohan's son Théodred was killed when Gríma Wormtongue refused to send reinforcements. Despite Théodred's death a company of the Rohirrim under Grimbold attempted to hold the fords, but they had to give them up against a force which outnumbered them more than five times.
The surviving Rohirrim fled south to the Hornburg, just nearly outrunning Saruman's forces, and almost doubled the number of defenders during the Battle of the Hornburg.