Short answers, pulled from the story.
The Sasanian Empire began in 224 AD when Ardashir I defeated Artabanus IV at the Battle of Hormozdgan. Ardashir crowned himself shahanshah to establish a new capital at Ctesiphon after his father Papak seized power in Khir by 200 AD.
Sassanian cavalry included two types of heavy units called Clibanarii and Cataphracts supported by light horsemen and archers. Infantry known as Paygan formed the bulk of foot soldiers recruited from peasants to guard baggage trains and storm fortifications.
Khosrow I, also known as Anushirvan, ruled from 531 to 579 and oversaw the empire reaching its greatest territorial extent. His rule controlled all modern Iran and Iraq plus parts of Arabia, the Caucasus, Central Asia, and India.
Yazdegerd III fled eastward to Khorasan where he was assassinated by a miller in Merv in late 651. The entire empire was absorbed into the Islamic caliphate within five years following this event despite repeated revolts in cities like Rey, Isfahan, and Hamadan.
Zoroastrianism became the official state religion under Khosrow I despite his tolerance toward Christian sons. Shapur II completed the collection of Avesta sacred texts while punishing heresy and apostasy throughout the empire.