In the AU mod, you can't 'build' armies. Instead, the Military Academy 'spawns' them like the SoZ spawns Ancient Cavalry units.
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"As far as general advice on mod-making: Go slow as far as adding new things to the game until you have the basic game all smoothed out ... Make sure the things you change are really imbalances and not just something that doesn't fit with your particular style of play." - WesW