Altena, a town of Westphalia, Prussia, on the Ruhr and Sieg railroad, in the district and 17m. W. S. W. of Arnsberg; pop. in 1871, 7,122. It is situated in a beautiful valley surrounded on all sides by high mountains. The town has for centuries been the seat of a flourishing industry, comprising numerous manufactories of iron and steel wares. Near by, on a high cliff, is the castle of the old counts of Altena (later counts of the Mark), which now belongs to the order of St. John.