Dol Guldur

Dol Guldur is located on the hill called Amon Lanc in the extreme southern reaches of Mirkwood. It was so far south in fact that it could be seen from inside Lothlorien. Due to Sauron's influence here, the forest became increasingly tainted with evil influences. These are what led to the name of Mirkwood being applied to the forest.

Purpose / Function

The fortress was built for many reasons. The first was to provide Sauron with a place to hide when he could not return safely to Mordor. This allowed him to rebuild his strength and bide his time. The strength in question also meant the rebuilding of armies. Which he used in ineffectual and repeated assaults on Lothlorien and Mirkwood.
Sauron could technically have hidden in a wide range of areas, Amon Lanc provided one major benefit - proximity to the river Anduin. This river was the last know location for the One Ring. So by making this his base, it allowed easier searching of that area. It also helped control the transit though the area that the Fellowship would have to later pass. This suggests to me that concealment was the real intent. So long as no one suspects that the area was worth attacking, no one would. This secrecy would then allow Sauron to send out raids and scouting parties to look for the One Ring. So in this sense, it is quite effective.
But it suffered from later alterations in intent. Once Sauron switched from covert and intelligence operations to more overt military ones, Dol Guldur became the base for his armies in the region. But no upgrades to the design appear to take place. Granted, the destruction of the One Ring led to a degradation of Mordor's military power in all aspects. But a solid fort can hold out even with a weak garrison for longer than what we see in the books. To a certain degree, this makes sense for a power acting in the Cult of the Offensive. Though a switch in purpose should have motivated changes. After all, Sauron should have had enough manpower available if he also had enough troops to attack with. The only reason this was not fatal to him was this proved to be a secondary theater.

Architecture

In the Hobbit movies, it is shown as a huge castle in ruin. So ruined that it appears to be unable to seriously withstand a siege. The magic seen in those movies to hide the fact it was occupied seems to be the only reason it had not fallen. After all, a weak fort occupied by a mortal enemy would naturally be a tempting target.
We see this play out in the War of the Ring. Once the One Ring is destroyed, so too was the Ring Wraiths protecting it and we can assume the magic hiding it. Which made it much easier to take. This siege lasted a few days and Galadriel destroyed it by herself. Yes, she is one of the most powerful beings in Middle Earth. But this is a fort built by the same guy who made Barad-dur - a fort that lasted 7 years under the right conditions. I would have expected Dol Guldur to at least be able to handle a month long siege even with a modest garrison.

History

Sauron took over the site in secret in 1100 of the Third Age. Though something as evil as Sauron could not keep the impact of his occupation secret for long. Apparently, he led to the spread of evil creatures and darkness covering the region. At first is t was assumed that a human sorcerer was living there. Others theorized one of the Ring Wraiths took up residence. Gandalf came to investigate. Sauron anticipated this move and evacuated before Gandalf arrived. Gandalf's suspicions were not exactly calmed even if they were not confirmed. The White Council was formed to continue the investigations and plan for Sauron's return.
Gandalf returned again in what became a much more fruitful trip. This time he conclusively proved Sauron had returned and was the one occupying the area. He also found Thrain - Thorin's father. From him, Gandalf gained the map and key needed to retake the Lonely Mountain. Anyone who has read The Hobbit knows the importance of that. After a delay due to Saruman's betrayal, the White Council attacked and rove Sauron away. But he had enough time to secure his power in Mordor. So he returned there and openly declared his presence.
But no garrison was left. Which allowed Sauron to reoccupy the site. Three Ring Wraiths were sent there to take charge of the search for the One Ring. They also built the forces used to attack Lothlorien and Mirkwood. This was successful in preventing those realms from aiding the other free people, but did little more than that. While fiercely fought, the elves won all four battles and drove Mordor's forces back. The combination of military power and the destruction of the One Ring led to Dol Guldur's final capture.

Type

Fortress

Owner

Sauron