Final answer:
Persepolis is the most important architectural achievement from the Persian Era. It was the seat of government and featured a unique and innovative design.
Step-by-step explanation:
The most important architectural achievement from the Persian Era is Persepolis. Persepolis was founded by Darius the Great in 518 BCE and was the seat of government and a center for ceremonies and festivals. It had a unique architectural design with a terraced plan and a lightweight roof of wooden lintels. Persepolis showcased the grandeur and power of the Persian Empire.