[Sandro]

Going back to the beginning, how to you propose to evactuate a city of thousands? Civilians travel slow and are easy targets. Sauron would find out that Minas Tirith had been evacuated and would send his army in fast pursuit. The walls were what saved the forces of Gondor. Without them, the army would have been slaughtered. The orcs would have no need for siege engines, and thus would move faster. Nazgul could harry the retreat. Depending on the time frame, saruman's forces could be a factor. Where do they retreat to? Edoras? Too small and lightly fortified. The Hornburg? They would be starved into submission. No, the descision to remain in the White City was the only choice available. Anything else would have resulted in untold destruction and death.