(Note: we won't talk about tactics, or applying this phalanx formation in battle, in this post. That will come in the next post!)
The hoplites constituted the heavy infantry of ancient Greece. The word stems from hoplon (Gk: ὅπλον) which describes the circular shield they carried.
A typical 4th century BC hoplite soldier is doodled on the right.
They carried a Doru (Gk: δόρυ), which is a spear roughly 9 feet long (2.1 meters). Regarding the doru:
The primary weapon of the hoplite, the dory spear was 7 to 9 feet in length, weighing 2 to 4 pounds, having a two inch diameter wooden handle, and tipped with an iron spearhead on one end and another iron tip on the other. The spearhead was often leaf-shaped, and the iron cap on the other end, called the sauroter (literally "lizard-killer") was often square in cross section, and was a counterbalance and a second deadly point on the weapon. This counterbalance function is essential, as the spear was handled with a single hand in the Greek phalanx formation.The Academy of European Swordsmanship 3 (2): 1. 2007.
So, in other words, the doru was not just a spear: its handle was also a deadly weapon. Thus if the shaft broke, the hoplite still had a melee weapon.
The strength of hoplites came in numbers, specifically when it takes the form of a phalanx.
Homer first described the hoplite battle line using the word "phalanx" (Gk: φαλαγξ) contrasting it with individual duals between characters.
The phalanx formation combines the concepts of a shield wall with a spear hedge.
As a shield wall, it was peculiar in that the shield covered the man to the left [a source of the term "right-hand man" referring to the person in the phalanx which is shielding another]. This proved to make the right flank particularly weak, since no one shielded them!
Usually some light infantry or cavalry was placed on the right flank to protect against flanking actions.
How was the phalanx as a spear hedge? With the doru, however, only two ranks can form the spear hedge.
Did this prove useful? Yes and no.
At the battle of Marathon, the phalanx formed was 4 men deep. (Herodotus said
The center, where the line was weakest, was only a few ranks deep, but each wing was strong in numbers VI.111)
But it was the center of the battle line which was badly beaten; the flanks' numerical strength carried the day. Herodotus remarks:
They fought a long time at Marathon. In the center of the line the foreigners prevailed, where the Persians and Sacae were arrayed. The foreigners prevailed there and broke through in pursuit inland, but on each wing the Athenians and Plataeans prevailed.Herodotus' Histories, VI.113
So how deep was the phalanx, usually?
Well, Xenophon notes that
Such was the cavalry of either antagonist. The heavy infantry of the Lacedaemonians, it is said, advanced by sections three files abreast, allowing a total depth to the whole line of not more than twelve. The Thebans were formed in close order of not less than fifty shields deep, calculating that victory gained over the king's division of the army implied the easy conquest of the rest (Hellenika VI.4.12)
So, the Spartans had 12 ranks deep, whereas the Thebans had 50 ranks deep.
The short answer is: it varied. Here the Thebans had an unusually deep formation, so I would speculate the Spartans had the "standard depth"...implying the standard depth is around a dozen ranks or so.
The Cambridge Ancient Histories, vol. 6, remarks phalanxes were "drawn up eight, twelve or sixteen ranks deep" (pg 680).
The phalanx formation had its short comings. For example, it was extremely vulnerable to skirmishers if left alone.
Indeed, this is what occured to the Spartan Phalanx at the Battle of Lechaeum (391 BC). Xenophon gives us the only record of the battle (recall that "Lacedaemonian" means "Spartan", and a Peltast is a Javelin thrower):
Now the tragic fate which befell the division was on this wise: It was the unvaried custom of the men of Amyclae to return home at the Hyacinthia, to join in the sacred paean, a custom not to be interrupted by active service or absence from home or for any other reason. So, too, on this occasion, Agesilaus had left behind all the Amyclaeans serving in any part of his army at Lechaeum. At the right moment the general in command of the garrison at that place had posted the garrison troops of the allies to guard the walls during his absence, and put himself at the head of his division of heavy infantry with that of the cavalry, and led the Amyclaeans past the walls of Corinth. Arrived at a point within three miles or so of Sicyon, the polemarch turned back himself in the direction of Lechaeum with his heavy infantry regiment, six hundred strong, giving orders to the cavalry commandant to escort the Amyclaeans with his division as far as they required, and then to turn and overtake him. It cannot be said that the Lacedaemonians were ignorant of the large number of light troops and heavy infantry inside Corinth, but owing to their former successes they arrogantly presumed that no one would attack them. Within the capital of the Corinthians, however, their scant numbers--a thin line of heavy infantry unsupported by light infantry or cavalry-- had been noted; and Callias, the son of Hipponicus, who was in command of the Athenian hoplites, and Iphicrates at the head of his peltasts, saw no risk in attacking with the light brigade. Since if the enemy continued his march by the high road, he would be cut up by showers of javelins on his exposed right flank; or if he were tempted to take the offensive, they with their peltasts, the nimblest of all light troops, would easily slip out of the grasp of his hoplites.
With this clearly-conceived idea they led out their troops; and while Callias drew up his heavy infantry in line at no great distance from the city, Iphicrates and his peltasts made a dash at the returning division.
The Lacedaemonians were presently within range of the javelins. Here a man was wounded, and there another dropped, not to rise again. Each time orders were given to the attendant shield-bearers to pick up the men and bear them into Lechaeum; and these indeed were the only members of the mora who were, strictly speaking, saved. Then the polemarch ordered the ten-years-service men to charge and drive off their assailants. Charge, however, as they might, they took nothing by their pains--not a man could they come at within javelin range. Being heavy infantry opposed to light troops, before they could get to close quarters the enemy's word of command sounded "Retire!" whilst as soon as their own ranks fell back, scattered as they were in consequence of a charge where each man's individual speed had told, Iphicrates and his men turned right about and renewed the javelin attack, while others, running alongside, harassed their exposed flank. At the very first charge the assailants had shot down nine or ten, and, encouraged by this success, pressed on with increasing audacity. These attacks told so severely that the polemarch a second time gave the order (and this time for the fifteen-years-service men) to charge. The order was promptly obeyed, but on retiring they lost more men than on the first occasion, and it was not until the pick and flower of the division had succumbed that they were joined by their returning cavalry, in whose company they once again attempted a charge. The light infantry gave way, but the attack of the cavalry was feebly enforced. Instead of pressing home the charge until at least they had sabred some of the enemy, they kept their horses abreast of their infantry skirmishers, charging and wheeling side by side.
Again and again the monotonous tale of doing and suffering repeated itself, except that as their own ranks grew thinner and their courage ebbed, the courage of their assailants grew bolder and their numbers increased. In desperation they massed compactly upon the narrow slope of a hillock, distant a couple of furlongs or so from the sea, and a couple of miles perhaps from Lechaeum. Their friends in Lechaeum, perceiving them, embarked in boats and sailed round until they were immediately under the hillock. And now, in the very slough of despair, being so sorely troubled as man after man dropped dead, and unable to strike a blow, to crown their distress they saw the enemy's heavy infantry advancing. Then they took to flight; some of them threw themselves into the sea; others--a mere handful--escaped with the cavalry into Lechaeum. The death-roll, including those who fell in the second fight and the final flight, must have numbered two hundred and fifty slain, or thereabouts. Such is the tale of the destruction of the Lacedaemonian mora.Xenophon's Hellenica
Book 4, Ch. 5
Note: I removed all footnotes and section numbers from Xenophon's quoted passage.