Gandalfs First entry to Dol Guldur

[Tinman]

I have been doing some research on Dol Guldur. I was interested to find out that when gandalf entered it and discovered the map+key, and the true identity of the necromancer, that this was actually his second time entering the tower.

From what i can find, the first time he entered the tower, Sauran fled to remain hidden. He then returned, and spend a few hundred(?) years gathering power again.

Does any one have any further details they can provide on this? I'm wondering:
1.What year did gandalf first enter Dol Guldur?
2. If sauron was not there, what did he find? Did all the orcs and wraiths clear out too? Where did they go?
3. Why did the Wise not tear down or reoccupy Dol Guldur at this time? From a strategic point of view... if there is a potential stronghold that can spell doom for all the surrounding freepeople, and will be extremely taxing to assault if occupied by enemy forces, why would you leave a for rent sign on it?
4. I want as much info on this as possible... so if you have any theories or facts, please let me know (no idea to small!)

[Valandil]

You can find answers to your first two questions (at least a full answer on the first and partial on your second) in Appendix B, Tale of Years - in the Third Age section, look at year:

2063 - Gandalf goes to Dol Guldor. Sauron retreats and hides in the East. The Watchful Peace begins. The Nazgul remain quiet in Minas Morgul.

The Appendices are wonderful things. Before you start on them, they may appear dry - but there's a wealth of great information and hints of all kinds of other stories left to be told. :)

[Lefty Scaevola]

I suspect that in 2063, Sauron's forces remained in control of Dol Guldar and southern Mirkwood, and just he retreated along with any personally identifying stuff to avoid being discovered to be Sauron (it was supected that the necromancer was one of the Nazgul.)
As for militarily controlling the area, I believe that Arnor had already fallen by that date, and Gondor had been battered by various invasions, plague, and civil war, and had no allies in and made no advances into southern Rhohavion or Mirkwood. No western expeditionary armies were to be had at the time.