The Last Bridge was the old stone bridge that carried the East-West Road over the River Mitheithel, about a hundred miles east of Weathertop. Its three arches crossed the wide river at the bottom of a valley, and to the east the land rose again into the wooded hills of the Trollshaws, in which Bilbo and his companions encountered an unfriendly trio of trolls. Indeed, there is an oblique reference to an 'ancient stone bridge' in chapter 2 of The Hobbit that seems to be a reference to the Last Bridge.
No record of its builders survives, but it was most likely constructed by the Dúnedain of Arnor, or perhaps of the later kingdom of Rhudaur, in which it lay. There was no other means of crossing the river for hundreds of miles to the north, and so the Bridge had great strategic value. In the War of the Ring, three Black Riders attempted to hold it against Aragorn and the Hobbits on their journey to Rivendell. Their plan was foiled by Glorfindel, who drove them away and left his token, a green beryl, on the Bridge.