Interesting, I was wondering about the same thing!
In my early game I have found this composition that suits my idea of a legion:
--3 Hastati
--3 Principes
--2 Triarii
--2 Velites
--2 Cavalry
After Marius reforms:
--10 legionary cohorts
--2 cavalry
These "legions' are joined in big battles by a separate auxiliary corps, usually:
--2 Auxillia
--1 or 2 archer auxillia
--2 cavalry auxillia,
under the command of 1 unit of heavy cavalry
I would like to see what other people use!
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