How could the Nazgul take Minas Morgul?

[Rune Son of Bjarne]

I am like the rest quite pussled by this question.

Gondor was strong so how could the Nazgul take it at all and why was it not re-conqured again?

Of course the Nazgul could have taken the city using the element of suprise or something cunning like The Spanish Incuisition and Boromir. But Tolkien tells us that there was a 2 year siege. . .now this implies that the Nazgul most have had a major military force as it takes a bit more than 9 wights and 1 spider to succsesfully lay a siege. Where did they get this force? It does not seems like there was many orcs in Mordor. . .I kind of figured it was men from Harad they used, but somehow I think Tolkien would have told us that.

How on eart could Gondor allow a 2 year seige on one of their major cities where one of the seeing stones were placed? No other war could have been more imperative!? and 2 years should be long enough to gather an army to defeat the Nazgul. Unless the Nazgul was to powerfull for any army when together, but then why did they not take all of Gondor?

Could it be that this is actually a part of the storry Tolkien did not think through?
I am led to belive that he just wrote down some nice story with out thinking down in detail that would fit with the overall picture. I know this is very unlike Tolkien, but I can think of no other possibility. It just doesn't add up.

Sorry for using so much space on saying nothing. . .