Let me get something straight, Minas Tirith is a city/fortress. Mordor is a realm of Arda. In my experience, a seige is an attack on a fortress (or somethig of the sort), so it would have to be quite an epic battle for Mordor to be "seiged".
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Sing my world into being, Ainur. The universe needs your power!
well mordor is kinda like a fortress in the way that the mountans around it just happen to surond three sides of mordor and I'm guessing that the mountans are impassable
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You will be in awe of my knowlage of the Middle Ages!
Well there is one way besides flying over the Ash Mountians. The Path of Cirith Ungol. But Shelob the evil spider guards that path so that is a hard one to get through.