Dol Guldur (Sindarin: "Hill of Sorcery") was Sauron's stronghold in Mirkwood in the fictional world of J. R. R. Tolkien's Middle-earth. It is first mentioned (as "the dungeons of the Necromancer") in The Hobbit. The hill itself, rocky and barren, was the highest point in the southwestern part of the forest. Before Sauron's occupation it was called Amon Lanc "Naked Hill". It lay near the western edge of the forest, across the Anduin from Lothlórien. In a passage that appears to apply the name Dol Guldur principally to the fortress rather than the barren hill it rose above, the company of the Ring first catch sight of it from Cerin Amroth in Lórien.
In the Second Age, before Sauron occupied the hill, Silvan Elves of the Woodland Realm under Oropher, father of Thranduil, populated the area of Rhovanion around Amon Lanc, but they withdrew northward, evidently to avoid conflict with Lórien and Moria. The Wise perceived evil growing there early in the Third Age, starting about T.A. 1100. During the latter part of the Third Age, Sauron used this fortress as a base to attack Lothlórien and the surrounding area. Tolkien suggests that Sauron settled on Dol Guldur as the focus for his rise during the period before the War of the Ring in part so that he could search for the One Ring in the Gladden Fields just up the river.
Read more about Dol Guldur: Etymology, Adaptations