ILSENBURG, a village and health resort of Germany, in Prussian Saxony, romantically situated under the north foot of the Harz Mountains, at the entrance to the Ilsethal, 6 m. N.W. from Wernigerode by the railway to Goslar. Pop. (1900) 3868. It has an Evangelical church, a modern château of the princes of Stolberg, with pretty grounds, and a high grade school, and manufactures metal wares, machines and iron screws and bolts.
Owing to its charming surroundings and its central position in the range, Ilsenburg is one of the most frequented tourist resorts in the Harz Mountains, being visited annually by some 6000 persons. The old castle, Schloss Ilsenburg, lying on a high crag above the town, was originally an imperial stronghold and was probably built by the German king Henry I. The emperor Otto III. resided here in 995, Henry II. bestowed it in 1003 upon the bishop of Halberstadt, who converted it into a Benedictine monastery, and the school attached to it enjoyed a great reputation towards the end of the 11th century. After the Reformation the castle passed to the counts of Wernigerode, who restored it and made it their residence until 1710. Higher still, on the edge of the plateau rises the Ilsenstein, a granite peak standing about 500 ft. above the valley, crowned by an iron cross erected by Count Anton von Stolberg-Wernigerode in memory of his friends who fell in the wars of 1813–1815. Around this rock cluster numerous legends.
See Jacobs, Urkundenbuch des Klosters Ilsenburg (Halle, 1875); Brandes, Ilsenburg als Sommeraufenthalt (Wernigerode, 1885); and H. Herre, Ilsenburger Annalen (Leipzig, 1890).