When I am playing as the Romans, I don't use missile units since their infantry can double as short-range archers (which, in an ideal battle, they always will). I follow what seems to be historically correct: Hastati form the front line, which is the longest (largest in terms of men); Principes form the second line, which is smaller, and Triarii form the third (which is usually much smaller since Triarii take two turns to train so I rarely use them... don't really need to). According to the unit information pages, the Hastati were the youngest members of the army and served to soften the enemy up and wear them down. The Principes then took the fight and finally the Triarii.. if they were ever required (I thought it was interesting that "going to the Triarii" or something like that was a saying back then meaning the battle was about to reach the climax). A typical battle has me forming up the lines and manuvering to match the CPU; usually they just stand there (I hope the AI patch coming soon is as good as rumored) so I move up just in range of my Hastati's pilum (don't know the plural.. pilae?). Once in range I allow fire at will at which point the Hastati unload their pilae, softening the stationary or advancing line. This usually provokes the enemy to charge. The Hastati remain in their line and meet the enemy charge. This holding maneuver allows the second line, the Principes, to fire at will and mow down a significant portion of the busy enemy attackers with their pilae while the enemy is busy attacking the Hastati. This helps a lot when facing phalanx units. At that point the enemy is either mostly dead or reinforcing itself and the Principes charge through the Hastati line. I usually place the Triarii on the flanks since they're effective at killing cavalry. Of course, when fighting phalanx armies you need a sizeable cavalry force. Have infantry hold the block in place and then flank with a charge to the side or preferably back.. should rout them in no time.
When playing phalanx armies I use the missile units to slaughter the oncoming army while the phalanx blocks hold them in place. It's very hard to maneuver the phalanx units so most of the killing is done by missilers and cavalry. I love defending a town with phalanx units.. the AI just pours troops at them. I don't think I've ever lost a town defense with phalanx units because they're almost impossible to flank when placed correctly in cities.. and they kill almost everything that dares approach from the front (except elephants, probably).
I'm playing as Carthage currently and plan on testing a strategy that is a bit of a combination between Rome and the phalanx armies.. since Carthage gets Libyan spearmen I can use them in the front line in phalanx blocks and while the enemy is busy attempting to kill them have Iberians or Scutarii flank them as opposed to having missilers attack.
__________________
In order to understand recursion, one must first understand recursion.