Aggressive, frontal charge of heavy cavalry (lance-wielding cataphracts) was the main tactics of the Sassanid Persian army, also in battles against Rome.
The typical deployment of the Sassanid army in battle consisted of three lines.
The first line (deployed in the front of the entire army) consisted of elite heavy cavalry, cataphracts (Savaran) in the center of the line, supported by medium and light cavalry (including horse archers - who, however, were not as important in Sassanid army as in Parthian army before) on both flanks. Heavy cavalry was usually deployed in a line of between 3 to 5 ranks deep. The proportion of heavy cavalry to medium and light cavalry in Sassanid period was like 50/50 (compared to 10/90 in Suren Clan's private army at Carrhae in 53 B.C., during the Parthian period).
The second line (deployed behind cavalry) consisted of infantry, equipped with long spears, swords and large shields (soldiers in first ranks also had armour - in further ranks usually not). The second line was further supported by war elephants, deployed between infantry units.
The third line (behind infantry) consisted of foot archers.
The battle tactics of Sassanid armies was extremely offensive and aggressive.
The battle was beginned by Sassanid horse archers and light and medium cavalry, who were harassing enemy lines with arrows, javelins, conducting "hit and run" attacks.
This "missile preparation" was then followed by a massive frontal charge of heavy cavalry (Savaran / cataphracts) against the very center of enemy line. At the beginning of the charge (while already advancing towards the enemy, at first slowly, then gradually increasing the speed) heavy cavalry was reforming its formation from a line (3 to 5 ranks deep) to a wedge formation (or rather a wedge-column formation) The center and the front of the wedge (wedge-column) formation consisted of heavy cavalry, while medium and light cavalry was joining that wedge and consisted both sides of the column (while cataphracts - as already mentioned - were deployed in the front - forming the wedge itself - and in center of the column, that followed the wedge).
Depending on a particular battle, this massive cavalry charge could be carried out either by center and both wings of cavalry, or by center itself (often left wing was being kept in reserve).
The goal of this charge was to destroy the center of the enemy army and / or to outflank the enemy.
If the first strike of heavy cavalry did not decisively smash the enemy center, cataphracts could detach from the enemy, withdraw, regroup / reform and then charge again (everything done under covering fire of horse archers, who were harrasing the enemy while the cataphracts were withdrawing and regrouping for another charge, as well as preventing the enemy from chasing the cataphracts while they were doing these maneuveres).
In case if several, repeated charges did not produce a desired effect, commander of the Sassanid army was sending the second line (infantry and war elephants) to charge the enemy army and to support the cavalry. Infantry and war elephants were attacking in a line formation (aka bench formation), elephants were in intervals between infantry units.
Sassanid infantry was a defence-oriented formation of spearmen equipped with long spears, swords and shields (and some of them also wore armour as I already mentioned - these were considered as "heavy infantry", while the remaining ones were "light infantry" - the only difference between them was armour, both had similar weapons). They were not designed for attacking alone - but they were trained to support cavalry in its charges. Another role of infantry was also to protect elephants from enemy attacks. Elephants were the main striking power of the 2nd attack wave of the Sassanid army (the main striking power of the 1st wave were cataphracts - in the 1st wave the support role and protective role was played by light cavalry, skirmishers and horse archers, while in the 2nd wave the support and protective role was played by infantry).
There were some - but not numerous - elite units of infantry.
Infantry was also supposed to exploit occassions created by charges of cavalry and elephants - in other words - infantry was going to charge into / penetrate into gaps in enemy lines "chopped" / "smashed" by heavy cavalry and elephants. Combined efforts of heavy cavalry, infantry and elephants were supposed to shatter enemy lines into isolated "nests of resistance".
In some battles, Sassanids used a slightly different tactics - in this modified variant, they were attacking with cavalry and infantry at the same time (instead of bringing infantry in only after cavalry proved unable to win the battle all by itself, as in the most common variant).
Third line (echelon) of the Sassanid army, as mentioned, consisted of foot archers. They were conducting fire before the hand to hand combat and also in case of covering withdrawal of their own fellow forces (be it cavalry, infantry or elephants) as well as sometimes they were also shooting to retreating enemy forces (when there was no cavalry available at hand to chase them).
During hand to hand combat of Sassanid cavalry and infantry, Sassanid foot archers were halting their fire, because there was a policy of avoiding "friendly fire" casualties.
The Sassanid king or other commander in chief, during the battle stayed in the rear area of his army and observed the battle from his command post - preferably located on top of some hill. He was entering the battle only in case of emergency situations, leading his most elite units - personal guards (Pushtigban) consisting mostly of heavy cavalry and elite foot archers (Kamandaran), as well as the elite division of Immortals (Zhayedan) who consisted mostly of heavy cavalry.
Commander of Pushtigban was at the same commander of a special division of the Sassanid army consisting of picked men from the entire army - war heroes and most distinguished veterans - called Gyanavspar, also known as Peshmerga (they were also kept in reserve and could be used to save the day and to turn the tide of battle only if absolutely necessary). They probably consisted of elite units of all kinds (heavy and light cavalry, heavy infantry, etc.).
Sassanid foot archers had heavy bows and large shields, but probably no armour (only helmets).
Apart from regular infantry (heavy and light infantry equipped with long spears, swords and shields), Sassanids also had levy infantry called Paighan. These were low-quality units drafted from peasants, equipped with shields and spears (no swords or armour). They were used for various tasks - protecting baggage train, as crews of catapults and other artillery, for transporting things, for building fortifications, camps and siege equipment. They were also used for sapping and as "cannon meat" while assaulting fortifications, which is why they often constituted even 2/3 of entire army when such an army was ordered to capture a fortified city or a fortress.
It should be noted that the Sassanid army put its reliance heavily on effectiveness of their shock cavalry charge. Heavy cavalry was the most important part of the Sassanid army - if cavalry failed, infantry was usually not able to win the battle alone, because Sassanid infantry was considerably inferior to Roman infantry. The Romans - on the other hand - usually relied more on infantry.
SOURCES (for the description of Sassanid battle tactics presented above):
Books:
T. Szeląg, "Amida 359"
K. Farrokh, "Sassanian Elite Cavalry"
M. Wozniak, "Armie starożytnej Persji" ("Armies of Ancient Persia")
P. Wilcox, "Rome's Enemies (3). Parthians and Sassanid Persians"
Not attacking, just puzzled.
