Simplify the following text for a Grade 12 native English-speaking person.
Original: The Sassanid era , during Late Antiquity , is considered to have been one of Persia 's \/ Iran 's most important and influential historical periods , and constituted the last great Iranian empire before the Muslim conquest and the adoption of Islam .
Simplified: The Sassanid era is considered to be one of the most important and influential historical periods in Iran . In many ways the Sassanid period saw the highest achievement of Persian civilization , and constituted the last great Iranian Empire before the Muslim conquest and adoption of Islam .