Dol Guldur:

The second stronghold of Sauron. Dol Guldur was located in Mirkwood and was the probable reason that the forests name was changed from Greenwood the Great. The fortress was probably built between the years of 1000 and 1100 of the Third Age. During the War of the Ring the fortress was home to one of the Nazgûl. It was completely destroyed by the forces of Lothlorien after the end of the War of the Ring.