The land and forests encompassing Amon Lac appear to have been constantly part of the realm of the Silvan Elves from the start of the First Age until the second millienium of the Third Age. Aside from some bands of woodsmen, the race of Men seem to have avoided, or at least ignored the area, as it lay outside the reach of Numenor (later, of Gondor or Arnor). The same seems true of the Dwarves, who besides building a trade road through, preferred to stay in their mountain halls.
Then suddenly into the tale appears the Fortress of Dol Guldur Was it a mini-Barad-Dur, or a hole in the ground there lived a corrupted Maiar? It IS described as a 'fortress', it had dungeons, and was large enough to serve as a garrison for a force of orcs strong enough to threaten both Lorien and the Woodland Realm simultaneously. Did Sauron and his servants construct a new entity, or simply occupy an existing one dating from earlier history?