: How could the army of the Nazgul have taken Minas Ithil? Well
: their first assault would have had the advantage of surprise.
: Once inside the Morgul Vale in force they would have been
: difficult to dislodge. All the evidence points to Sauron's
: forces being very adept at seige-works, and orcs were used to
: tunneling and spade-work. In the time (weeks - a month?) it
: would have taken for Earnur to assemble a viable army the enemy
: could have raised circumvallation earthworks and pallisades,
: trenches and saps, and have been able to fortify the approaches
: to the city up the Vale (see the description of Frodo and Sam's
: struggle up the Vale, and how easy it would have been to
: defend). Once in place the Nazgul's army would have been as
: difficult to assault as WWI trench defences on the Western
: Front. No doubt the Gondorian army made the attempt, but once
: repulsed Earnur would have been faced with a dilemma, even if
: he reached the city his army might have been decimated, in
: which case he would have had little choice other than to
: evacuate the city as being no longer defensible. In this
: scenario he would have achieved nothing other than sparing the
: lives of the citizens and garrison , but have destroyed the
: army of Gondor and left the state open to assault from Rhun or
: Harad in the process. Earnur may have decided that the only
: reasonable course to take would be to raise counter defences at
: the mouth of Morgul Vale and hope that these could act as a
: stopper to keep the Nazgul bottled up in Minas Ithil. The
: long-term future of Minas Ithil, even if relieved, in the face
: of an actively hostile power in control of Gorgoroth may have
: appeared bleak.

It's doubtful Earnur would have brought up the entire army of Gondor in any event. The Easterlings were still a threat, and the Haradrim as well. Earnur probably could have mustered maybe 20,000 men for a counterassault against the Nazgul's army, but the defenses you suggest could have been thrown up quickly, if manned by only half that many troops, would have been sufficient to keep Earnur's army away from the city.

Minas Ithil may actually have been in control of the pass from Mordor before the Nazgul led their army over the mountains. As someone pointed out earlier, treachery may have played a role in that war, and if so then why not in subverting Gondor's defenses. I doubt the soldiers of Minas Ithil simply ignored Mordor altogether. They just couldn't keep a full watch over it.

Minas Ithil therefore had strategic importance to Gondor, in that it provided access to Mordor through which patrols could be sent to spy out the land. For that reason alone it would have been imperative for Sauron to seize the city and so block Gondor's access to Mordor, even though he might also have been seeking knowledge of the One Ring.

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