Edoras was indefensible. The stronghold for Eastern Rohan was at Dunharrow- where even a small force could hold off an army indefinitely, given sufficient provisions.
But trying to maintain a line of communications across the open plains, in the face of a vast Orc-army? Suicide; a pointless sacrifice of manpower. With the halt/lame/old/boy/women forces at hand, you need every force-multiplier (i.e. walls and cliffs) you can get.
Some communications (message, not logistical) could be maintained between Helm's Deep and DH vis the mountain-paths (that's still about 4 days). But then Helm's Deep wouldn't be under much immediate threat, Saruman and the Dunlendings having been neutralized.
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The entire plot of The Lord of the Rings could be said to turn on what Sauron didn’t know, and when he didn’t know it.
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