Pezhetairoi are the classic Alexandrian phalanx, the anvil of the two part Makedonian system of warfare. They are defensive infantry par excellence.
The Pezhetairoi are the heart of the military machine of all the Diodachoi kindgoms. They are well disciplined and highly motivated pikemen that are armed and armored to the teeth. They are armored in a linen cuirass, a Thraikian cap, a bronze greave on the left leg, stout boots, good bracers, and reinforced shoulder pads made from hardened linen (due to their experience with the deadly curved swords of Thrace). They have Illyrian style round shields attached to their bodies by leather straps that help support the weight of the shield and keep their hand free to grasp the long and unwieldy sarissa. They are defensive infantry par excellence and are the anvil of the two part Makedonian system of warfare, the heavy cavalry being the hammer. They should be used to anchor enemy soldiers while the Thureophoroi harass the flanks and the heavy cavalry smashes into the flanks and rear.
Historically, the Pezhetairoi are the classic Alexandrian phalanx. They were used to great effect against the Persai, Medoi, Baktrioi, Indoi, Phoinikes, and many, many others. They are an effective force and have not changed much over the centuries. The Romaioi were able to defeat them as easily as they did for two main reasons. One, the Romaioi army was at a high state of readiness and tactical prowess after defeating the Karchedoi. Two, the heavy cavalry arm of the Diodochoi armies had degenerated to the point where they were no longer able to field significant numbers to fulfill their part of the hammer and anvil tactic of Alexandros. There were also many lesser reasons, numbering among them the misuse of the Thureophoroi, the under use of Peltastai, and the lax state of warfare that the Diodachoi states were used to. In any case, the phalanx was not as anachronistic or inflexible as widely believed; it was simply misused and under-supported. In the thirteenth century and onwards, pikemen in similar formations were able to work wonders with more capable generals and a better cavalry arm. Do not under appreciate pikemen, for they are still a war winning force.
The Pezhetairoi are an effective phalanx unit capable of holding the battle line if properly supported. They can also be used as an effective sword infantry in a pinch due to their excellent armor and good shields. Like all phalanx units however, they are very vulnerable to attacks from the shield-less flank or the rear and infantry units with armor-piercing weapons or high lethality will easily tear them to shreds in melee.