How could the Nazgul take Minas Morgul?

[CSteefel]

Originally Posted by Alcuin

The problem was, I think, that the Dúnedain of Gondor were first unprepared for any attack at all, believing that having vanquished both the Wainriders and the Haradrim, they faced no risk of attack; and in the second place, they had no idea what tactical problems they faced in combating the Ringwraiths.

Eärnil lacked the insight and experience to deal with the Nazgûl, something the commanders of Arthedain possessed in spades;

This seems to be the key--the Dunedain of Gondor were simply not prepared for an assault of this kind by the Nazgul. A conventional assault (with Orcs, for example) would have been unlikely to have succeeded. And as Alcuin has said, the Arthedain had considerably more experience in this department, plus they had the aid of the Elves. In the end it was Glorfindel who drove off the Witch King...