Passage: In T.A. 2000, the Nazgûl besieged Minas Ithil and, after two years, captured it and acquired its palantír for Sauron. The city thereafter became Minas Morgul, the stronghold of the Nazgûl. Sauron returned to Mordor in T.A. 2942 and declared himself openly in T.A. 2951. Two or three of the Nazgûl were sent to garrison Dol Guldur, his fortress in Mirkwood.

Question: What was the name of Sauron's fortress in Mirkwood?
Dol Guldur