A short video offering a brief description of the Roman army and how it set up both before and after the reforms by Gaius Marius.
Using the Rome Total War engine and the Total Realism mod for that to depict formations and fighting tactics.
Information regarding the Roman army pre- reforms comes from Polybius' descriptions during the Second Punic War (218 - 201 BC). Information about the professional army is more readily available, notable sources include Julius Caesar's Commentaries.
Adrian Goldsworthy's book "The Complete Roman Army" is a great reference for this sort of information.
Also, apologies for any incorrect information and my awful voice and editing....(to the billions I'm sure who will ever watch this)