Attacking from the left is not the same as attacking from the right. This is most obvious with missile weapons. Troops always hold their weapon in their right hand and if they’ve got a shield it is in their left hand. The defence versus missiles is dependent on what arc the unit is being fired at from.
From the frontal arc, defence vs. missiles is armour value + twice the shield value. From the left arc it is armour + shield value. But from the right it is only armour (since the shield is carried in the left hand). This means that given equal terrain, you should always prefer to place your missile units so they can shoot into the right arc of their target for more casualties. Of course the best is still the rear arc which only provides half the armour value for defence vs. missiles. Take a unit of Hastati as an example (5 armour and 5 defence shield). Defence vs. missiles would be front:15, left:10, right:5, rear:2.5.
You can also position other units in ways that maximise their effectiveness vs. the AI. One of my favourites is a method for killing generals. When a unit is shot at with missiles they usually give chase, especially if they are a strong unit that could defeat the unit they are being shot by. A Generals Bodyguard is the best example of this. You can use this AI preference for charging at missile troops to your advantage. Put the missile unit in front of a melee unit such as I explained in LESSON TWO. They can shoot the general and encourage him to charge. This is where placement is most important. A general (or captain) is positioned at the front on the left of a unit (except for a captain using wedge formation). This means you can position a unit in a position just ahead of where the Bodyguard is charging to as a means to directly engage the general. One of the most annoying parts of RTW is when you completely destroy an army only to have one general escape. This ensures this doesn’t happen. In the pic below you can see that the Scipii general is marching (not charging yet) for the Militia Cavalry that are throwing javelins at them. By placing my right phalanx slightly ahead I ensure that the general is caught in the spears and directly engaged. If however it had been the other way around and my left phalanx was forward the general would have been out of harm’s way.
Another bonus of this is that because the general will snick the first unit the Bodyguard will be denied the ability to charge. They will also start to wheel around to attack the first unit and then run into the second unit which will act as though attacking them from the side. This is one of the useful tactics you can use with the phalanx. Don’t always set it up in a straight immovable line (though this is useful sometimes too), be aggressive (these units had just finished moving there – they aren’t yet braced)
EDIT: An example of a perfect execution of this strategy. The general is completely isolated from his bodyguard in the background and fell in moments as a consequence.