Just to add a little history regarding Dol Guldur, find below a list of entries from the LotR Appendix B - Third Age:
- 1050- About this time a shadow falls on Greenwood, and men begin to call it Mirkwood.
- 1100 - The Wise (the Istari and the chief Eldar) discover that an evil power has made a stronghold at Dol Guldur. It is thought to be one of the Nazgul.
- 2060 - The power of Dol Guldur grows. The Wise fear that it may be Sauron taking shape again.
- 2063 - Gandalf goes to Dol Guldur. Sauron retreats and hides in the East.
- 2460 - Sauron returns with increased strength to Dol Guldur.
- 2850 - Gandalf again enters Dol Guldur, and discovers that its master is indeed Sauron, who is gathering all the Rings and seeking for news of the One, and of Isildur's heir.
- 2851 - The White Council meets. Gandalf urges an attack on Dol Guldur. Saruman overrules him.
- 2941 - The White Council meets; Saruman agrees to an attack on Dol Guldur, since he now wishes to prevent Sauron from searching the River. Sauron having made his plans abandons Dol Guldur.
- 2951 - Sauron sends three of the Nazgul to reoccupy Dol Guldur.
- 3019 - Three times Lorien had been assailed from Dol Guldur...and when the Shadow passed, Celeborn came forth and led a host of Lorien over Anduin in many boats. They took Dol Guldur, and Galadriel threw down its wall and laid bare its pits, and the forest was cleansed.
About time! So if I'm doing my math right, for 1919 years, give or take, Dol Guldur was occupied except for 10 of those years when Sauron feinted. Reading the Appendix, do you know how much happened in those almost 2,000 years?
How much better had Galadriel stepped across the River a few years sooner.