Altenstein, in Germany, is a castle upon a rocky mountain in Saxe-Meiningen, on the south-western slope of the Thuringian Forest, not far from Eisenach. It is the summer residence of the dukes of Meiningen, and is surrounded by a noble park, which contains, among other objects of interest, a remarkable underground cavern, 500 ft. long, through which flows a large and rapid stream. Boniface, the apostle of the Germans, lived and preached at Altenstein in 724; and near by is the place where, in 1521, Luther was seized, by the order of the elector Frederick the Wise, to be carried off to the Wartburg.