Normally 3-4 ranks deep, but as Kraxis said if Ihave an opportunity to get a unit to wrap around the flanks of an enemy I would maybe go for a 1-2 ranks.

As I understand it, it seems that thin lines of troops suffer quite badly if they're on the receiving end of a strong charge...

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We few, we happy few, we band of brothers, for he today that sheds his blood with me, shall be my brother be he ne're so vile, this day shall gentle his condition. And Gentlemen in England, now a bed, shall think themselves accursed they were not here and hold their Manhoods cheap, whilst any speaks that fought with us upon Saint Crispines day.

William Shakespeare - Henry V