Originally Posted by JeromeGrasdyke
The training is done in the standard Total War style, and the tech trees are simpler than Medieval - generally troops are built by one building, although buildings do sometimes need more than one other building before they become available. We did experiment with plugins for buildings and unit inheritance chains, but it felt a little 'niggly' to play and meant you had to spend a lot of time micromanaging it, away from the meat-and-drink of the game - the battles - which meant it got dropped.
But the main thing preventing all-Triarii armies is the fact that they're not a good idea; as with most units in TW, they have their nemesis as well, in decent non-spear infantry. So in the end the best army makeup is one with a good mix of troop categories, with the highest quality you can manage in each category - pre-Marius that would be Velites for skirmishing, Archers and Ballistae for missiles, Princeps for main infantry and Triarii as spearmen, with some Equites to provide cavalry support.
The way the Marius event works is through a dual tech tree - when the event comes along, a whole bunch of units are made obsolete, and their places in the tech tree are taken by legionary ones instead. Legions are raised a cohort at a time, and first cohorts are awarded by the Senate as rewards for completing tasks.