How could the Nazgul take Minas Morgul?

[Farael]

Originally Posted by Sir Kohran

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 think that the answer actually lies in your question. What could have taken two years to occur? and on the other hand, two years is far too long for a siege, food supplies may last for a few months, but I wouldn't think much more than that.

So here is my theory:

The nine Nazghul, along with a few other creatures of evil persuasion, started haunting the areas around Minas Ithil. Now, since there is not an army camped outside, all the might of Gondor might not do much of a difference. Furthermore, if Gondor did not have the resources to spare to launch a "turn every stone, search every cave" kind of attack to find them, it's likely that they would have sat there and hoped for the best, doing small skirmishes if they happened to find a specific hot spot. But this kind of warfare probably had negative effects on the population of Minas Ithil, psychological warfare if you need to give it a name. Eventually the population would flee, until the city would be weakened and no longer a "strong point" for Gondor. Once that happens, it is possible that the Nazghul, leading a smaller army recruited during those two years, would have been able to take the city.

That explains the two-year long "siege" and also how the Nine (with some help, but not necessarily a huge army) were able to take Minas Ithil.