Dol Guldur, "The Hill of Dark Magic"

Timespan: Roughly 1050 through 3019 in the Third Age
Location:The forest of Mirkwood

Dol Guldur is mentioned by name in The Fellowship of the Ring, as the fortress home of a being known as The Necromancer. It is revealed shortly after that this name was merely a facade maintained by Sauron as he rebuilt his powerbase.

History: Built by Sauron after his defeat at the hands of the combined forces of the elves and Nśmenoreans and the destruction of the first Barad-dur, Dol Guldur was the recuperatory point for the dark forces after the fall of their master Morgoth. Dwelling there under the pseudonym of "the Necromancer," Sauron nursed his wounds and plotted, until Gandalf of the Wise uncovered his true identity in 2941. For ten years, the fortress stood empty, until Sauron sent his Nazgūl to reclaim it.

The fortress is believed to have been destroyed in, or around, the year 3019, after the War of the Rings.