Blankenburg. I. A circle in the duchy of Brunswick, Germany; area, 183 sq. m.; pop. about 23,000. The southern part, bordering on the Hartz mountains, is covered with forests, and contains valuable iron mines and marble quarries; the northern part is fertile and well cultivated. Until the 12th century the district was known as the Hartingau; and it was subject to the counts of Blankenburg till 1599, when it passed into the possession of Brunswick on the death of the last of the Blankenburg house. In 1090 it was ceded to Ludwig Rudolph of Wolfenbtittel, and in 1707 it was made a principality. After being an independent government till 1731, it again passed into the possession of Brunswick, and remained subject to that duchy. II. The principal town of the circle, situated among the Hartz mountains, 14 m. E. of the summit of the Brocken, on a small stream of the same name, and near the foot of a picturesque mountain called the Blankenstein; pop. in 1871, 3,928. Near by is the palace of Luisenburg, which contains 270 apartments and a large collection of paintings; and at the distance of 1 1/2 m. are the ruins of the castle of Regenstein or Reinstein, hewn in part from solid rock.

In 1625 the town was besieged by Wallenstein. During the seven years' war the court of Brunswick had its residence here, the place preserving a neutrality which was respected by all parties. Regenstein was taken by the French in 1757, but retaken by the Prussians during the next year. Louis XVIII. resided at Blankenburg from 1796 to 1798, as the count de Lille.