Who built the fortress of Dol Goldur, and when was it built? And was it always a place for evil, or Sauron just occupied it?
Despite the fact that Mordor is much better fortified; artificially by the old fortifications, and geographically, as it is surrounded by the mountains. Also considering the presence of mount doom that seem to enhance Sauron's power, Sauron chose to start his campaign from Dol Goldur, despite Mordor being ready for quite some time.
Why did he choose this enigmatic fortress, and not go straight for Mordor or even go earlier?