From this we can easily conceive what is the nature and force of a charge by the whole phalanx when it is sixteen deep.
In this case those further back and the fifth rank cannot use their pikes so as to take any active part in the battle.
They therefore do not severally level their pikes, but hold them slanting up in the air over the shoulders of those in front of them, so as to protect the whole formation from above, keeping off by this serried mass of pikes all missiles which, passing over the heads of the first ranks, might fall on those immediately in front of and behind them.
But these men by the sheer pressure of their bodily weight in the charge add to its force, and it is quite impossible for the first ranks to face about.
Such being in general and in detail the disposition of the phalanx, I have now, for purposes of comparison, to speak of the peculiarities of the Roman equipment and system of formation and the points of difference in both.
Now in the case of the Romans also each soldier with his arms occupies a space of three feet in breadth,
but as in their mode of fighting each man must move separately, as he has to cover his person with his long shield, turning to meet each expected blow, and as he uses his sword both for cutting and thrusting it is obvious that a looser order is required,
and each man must be at a distance of at least three feet from the man next him in the same rank and those in front of and behind him, if they are to be of proper use.
The consequence will be that one Roman must stand opposite two men in the first rank of the phalanx, so that he has to face and encounter ten pikes, and it is both impossible for a single man to cut through them all in time once they are at close quarters and by no means easy to force their points away, as the rear ranks can be of no help to the front rank either in thus forcing the pikes away or in the use of the sword.
So it is easy to see that, as I said at the beginning, nothing can withstand the charge of the phalanx as long as it preserves its characteristic formation and force.
Walbank Commentary