SO ....
The only clue in the Unfinished Tales of the 2nd Age is (sorry if the translation isn't awesome)
Thranduil's realm strechted in the wood around the Lonely Mountain, which covered the western side of Long Lake, before the coming of dwarves and raids of dragons. This elven people emigrated from the South, where they were parent and neighbour of Lòrien elves ; but they have stayed in Greenwood the Great, at the east of Anduin.
According to this, south Mirkwood were inhabited by elves in the past. That is
highly possible that they settled around Amon Lanc, as it is said on many websites but I don't know if it is clearly mentionned by Tolkien or his son. In my mind, the elves built their capital
around Amon Lanc. Indeed, Amon lanc means "Bald hill" and silvan elves wouldn't settled in a place without trees. They could have digged tunnels and halls under the Hill, like Thranduil's caverns but even this one is described as "wooded hill".
To conclude, Sylvan elves possibly lived under hill but most probably around and afterwards Sauron erected Dol Guldur at the top. For Dul Guldur project, you should use a "
evil " style because Sauron wouldn't have kept his fortress elvish looking. As Minas Morgul, Cirith Ungol, Durthang, Morannon which were Gondorian outpost, he perverted it. The best example is Minas Ithil : it was surely a stunning city, white and Glittering like Minas Anor, and it became a dark and gloomy place.