The Royal Road or Achmaenid Road with its system of way stations and riders aided communication in Darius' empire. The road connected Susa with Persepolis and India.
https://www.thoughtco.com/royal-road-of-the-achaemenids-172590
A building structure does not unite an empire. The structure which united it was a system of good and fair government, with protection from internal and external threats.
The governing system of the Persian Empire, resting on traditional local government, provincial government and central imperial government.
Great King Darius Great King Darius
Darius further refined what Cyrus had previously established - Satraps who were essentially provincial governors . ~ See related link below .
Darius I led the Persians in the First Persian War. Xerxes I led them in the Second.
Cyrus the Great conquered the ENTIRE Middle East, Egyptian empire, and parts of India, China, Mongolia, Russia, and Greece. Xerxes the great conquered Greece and opened the door to Europe for Persia. Sassanid also was famous for conquering the far east. Also: Darius
Attempting to organise security and prosperity within the empire.
Under Xerxes I the Persian army captured Athens following the battle of Thermopylae. The Persian king to conquer Athens , was the famous Persian king called Cyrus.
King Alexander III (the Great).
(DARE-ee-us). The name Darius originally comes from the Persian Greeks. Meaning possessions well; upholder of the good; rich and famous. Naturally of a King. King Darius of Babylon, fifth century B.C. Mainly named for a boy. The name was most popular in the year 1991. The name is Persian. ;) - Darius Martin (Lloyd-D) Pompano Beach, FL
The royal road.
His rule over the Persian Empire in the early days of history.
Iran, once called Persia, is famous for its rich history and culture. Persian Empire was the first modern empire extended from west of China to east Europe. Also Iran is famous for Persian Caviar, Persian Carpet and Persian Cat.