The Parthians ruled over a multi-ethnic, multi-cultural empire.
It consisted of a mix of provinces ruled by governors and a number of semi-independent kingdoms and principalities.
They had a capital at Ctesiphon, though did not rely on it very much.
The Romans managed to conquer it three times, without bringing down the whole empire.
Instead of sitting on the throne, the Parthian royal court traveled around the country.
The Parthians had no standing army.
Instead they used a feudal system that could call up troops in time of need.
This made it very suitable for defense, but much less for offense.
The Parthians had to fight off rivals from the east, north and west.
The latter were the Romans and their battles with the Parthians are the best documented.
The most famous victory of the Parthians over the Romans was at Carrhae.
Almost all Parthian troops were cavalry.
The "āzāt" (nobility) provided heavily armored cataphracts,
the lower classes horse archers in the old steppe tradition of the Scythians.
The latter became famous among the Romans because of their ability to deliver a backwards firing 'Parthian shot'.
It seems that initially the ratio of heavy to light troops was about 1:10; later the number of cataphracts increased.
By the time that the Parthian empire was succeeded by the Sassanid, heavy cavalry was dominant.
Horse cavalry was supplemented by camels in small numbers; they were used more often in the baggage train than in battle.
They also had infantry, but it was weak and small in number.
The general Parthian battle tactic was to harass the enemy with horse archers, thinning their ranks and trying to lure them into charges.
When the enemy was sufficiently softened, the heavy cavalry moved in to smash their formation to pieces.
The lack of infantry and siege engines made the Parthians very weak in siege warfare.
The feudal system also meant that military campaigns had to be short and placed well in the summer season; at winter the warriors went home.
War Matrix - Parthians
Roman Ascent 200 BCE - 120 CE, Armies and troops