ILSENBURG, a village and health resort of Germany, in Prussian Saxony, situated under the north foot of the Harz mountains, at the entrance to the Ilsethal, 6 m. N.W. from Wer nigerode by the railway to Goslar. Pop. (1933) 5,333. It manu factures metal wares.
The old castle, Schloss Ilsenburg, lying on a high crag above the town, was originally an imperial stronghold probably built by the German king Henry I.