Actually shifty, i'm not positive but i think that you can't train units in under a turn, plus if you could mobilize your entire population you can essentially strip a city of it's inhabitants and make it useless to capture. And while peasents don't need very much training (maybe a week to show 'em which is the pointy end) you still have to make the spears and get a officer to lead 'em and feed them and all the other logistical problems assosicatied with large armies. In truth i like the idea of quickly training them but i think it's unfeasible.
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