Exceptionally I joined the Modding Contest on HeavenGames.com, naturally I don't want to keep my contestant back from you. ;)
This picture shows the Roman Princeps, it means »the first« and refers to position of the unit in the Roman battle lines. However, they were never really standing in the first line, in front of them were the Hastati and depending on the phase of the battle mostly skirmishers as well. Principes were, in view of the Roman Republic - the richer and better equipped Hastati. This is why the Princeps is the next level of the Hastatus, which will be available in Age III. So both units have the same abilities and functions. They are heavy infantry and have as secondary fight mode also a ranged attack. The effective Roman standard soldier.