you can position your light troops and cavalry on the flanks to cover them while your infantry lines roll up the enemy; or, you can cement you flanks (like Suetonius Paulinus against Boudicca) against something difficult to pass through (ideally cliffs, but it depends). And if the enemy has a longer line than you (you should be able to tell right after deployment) and you're really worried about being outflanked, do like Caesar at Pharsalus and form your third line to the side
(L being cohorts; C cavalry _ being gap)
C__L_L_L_L__C
__L_L_L_L
_L ______L
Now, lets suppose the enemy is massing cavalry on our left flank. What we do is put our line in this formation. When the enemy cavalry engages our left, we take our 2 cohorts and charge forward at the cavalry, bogging them down in a melee battle which is usually not a good thing for cavalry.
Now, about the enemy having a longer line. let's suppose that we're up against a phalanx. Phalanxes must be deployed close together or else the risk being outflanked. Because we deployed our legionary line with gaps in it, the enemy has eight units on the same front as our 4.
Quoted from harpoon:
And if they kill fast then why should you have to move lines back and forth to relieve tired units? They don't even get tired that fast.
No need to move lines back and forth, harpoon. You can just use fresh troops to supplement your forces, which causes the enemy to lose morale, as I believe Centurion Marcus says on a number of occasions.Now, I bet you're thinking that with gaps in our line, the enemy could just walk right through and attack our flanks and rear. Well, that's why our second line is positioned to cover the gaps. Also, for the enemy to outflank us, especially if they're phalanxes, they will have to walk around with their units. Phanlanxes have the nasty habit of putting their spears up when they are moving, leaving them vulnerable to a quick charge by the second line.
Have you tried using this system, Harpoon? I suggest you do. It really works.
His ego nec metas rerum nec tempora pono:
Imperium sine fine dedi.
(P. Vergilius Maro, Aeneid I. 278-79)
We are all, so far as we inherit the civilisation of Europe, still citizens of the Roman Empire, and time has not proved Virgil wrong when he wrote nec tempora pono: imperium sine fine dedi.
(T.S. Eliot)