It was said to be "in a dark hill" (Of the Rings of Power and the Third Age) in southwestern Mirkwood "nigh to the River" Anduin.

'There lies the fastness of Southern Mirkwood,' said Haldir. 'It is clad in a forest of dark fir, where the trees strive one against another and their branches rot and wither. In the midst upon a stony height stands Dol Guldur, where long the hidden Enemy had his dwelling. We fear that now it is inhabited again, and with power sevenfold. A black cloud lies often over it of late.' [Lothlorien]

There are prisons in the pits of Dol Guldur (Hill of Sorcery; Dol = Head, Gul = Sorcery, Dur = Dark) such as where Thrain was held for half a decade. Gandalf has referred to it as the "pits" or "dungeons" of Sauron and referred to how the Dwarf Ring "was taken with torment from Thrain in the dungeons of Dol Guldur." (The Council of Elrond)

He had been in the pits of Dol Guldur for five years at least. I do not know how he endured so long.... I think that the Dark Power had desired nothing from him except the Ring only, and when he had taken that he troubled no further, but just flung the broken prisoner into the pits to rave until he died. A small oversight. [The Quest of Erebor]

Dol Guldur was the lesser of Sauron's domains, but Gandalf told the Company,

I alone of you have ever been in the dungeons of the Dark Lord, and only in his older and lesser dwelling in Dol Guldur. Those who pass the gates of Barad-dur do not return. [A Journey in the Dark]

Dol Guldur did have some punch, it was a "stronghold" (The History of Galadriel and Celeborn) and it had been attacking both Lorien and the Woodland Realm before "fear and despair fell upon his servants and allies" (The Third Age). Sauron died and the forces of Lothlorien threw down Dol Guldur.

Celeborn came forth and led the host of Lorien over Anduin in many boats. They took Dol Guldur, and Galadriel threw down its walls and laid bare its pits, and the forest was cleansed. [The Third Age]

I suppose Dol Guldur was more than just pits and dungeons. It seems the only insight into Dol Guldur with Gandalf are in its pits where he ran into a nearly mindless Thrain, although he said he had other business there but I do not recall any mention of some other place in Dol Guldur. Where Sauron stayed, or his leiutenants, or forces, I'm not sure but it can't have been much different from the layout of any other place such as Minas Tirith, or Mordor. I'd say seek some insight there, into the movement of soldiers in Mordor and in Gondor, where they held prisoners, kept watch, and the seats of their lords. For example in Mordor

there were camps, some of tents, some ordered like small towns. One of the largest of these was right below them. Barely a mile out into the plain it clustered like some huge nest of insects, with straight dreary sheets of huts and long low drab buildings. About it the ground was busy with folk going to and fro; [The Land of Shadow]