[Sir Kohran]

I guess this is one of Tolkien's rare plot holes. I think what he does wrong here is that he intends for the story of the city's fall to be vague and unexplained, by not telling us what happened and so allowing us to come up with our own theories, but then he goes and uses very specific terms like 'siege' and 'two years' that leave us caught somewhere in the middle, unable to make up our own version or find a canon explanation.

I think it's actually fairly easy to explain the 'siege' term - the eastern lands of Middle Earth and the great wastes that lay beyond it were full of monsters and evil people that could be probably be swayed by the combined power of the Nazgul, and used as a makeshift army that allowed for a proper attack. I could see that happening.

What really confuses me is how the city itself fell. Perhaps if the attack had been sudden and the city seized before anyone had a chance to react, then it would be plausible, but the fact that it took two years seems very odd. Why didn't Gondor send reinforcements? Two years is more than enough time to gather a sizeable force to relieve the defenders. We could assume that the army that took Minas Ithil was too strong to be defeated by any Gondorian army, but then you end up wondering why such a mighty army didn't just take over all of Gondor, if it couldn't be defeated by the Gondorians' combined forces.

The idea that the Gondorians didn't see the city as all that important sounds very strange. One of the greatest cities of the south, and an important tactical base, being attacked by all the nine Nazgul? That would surely be a cause for serious concern.

Also, why did it take that long to subdue the city at all? In a battle, every single second counts for something important, so what could have taken two years to occur? And how did the defenders stay alive for so long? What was their food supply?

I suppose this is just one of those things that we'll never know...