Darius I not only collected large amounts of silver and other tributes from all provinces across the country, but also had a large royal slave estate directly under his own jurisdiction. According to the Pasaypolis clay tablets published in 1969, in the 22nd year of Darius's reign (i.e. 500 BC), the royal estate distributed grain (mainly to slaves) ("curtas"). is barley) totaling 16,212.7 boels (1 boer is equal to 10 liters), with a monthly average of 1351 boers; barley seeds retained are 1707 boers; barley distributed for livestock and bird feed*** 3818 boers; the flour distributed to officials was 2211 boers. From the discovered and published information about the expenses of the royal estate, we can see that the number of slaves, cultivated land area, livestock and management officials of the royal estate were quite large. The workers here, Kurtas, are mostly people of various ethnic groups conquered by Persia. They have no personal freedom, no property, and live by receiving supplies, but most of them have their own families. The Persian rulers could move the kurtas from one place to another to work at will. In addition to the royal estates, there were also aristocratic-bureaucratic slaveholding estates that also exploited the labor of the Kurtas. The large estates owned by the royal family and aristocratic bureaucracies were spread across the country and formed an important part of the economic structure of the empire.
The wealthy merchants who appeared in the Neo-Babylonian period - the Ajibe family and the Mulla tree family, etc., became even richer at this time. They set up banks in many cities to lend money to bureaucrats and nobles and earn large amounts of interest. They also gained unprecedented profits from tax farming and trade. For the Persian imperial government, its vast territory still made transportation and communications a stressful issue. Six thoroughfares were built, and stagecoaches for government messengers to ride along were prepared. Although the above measures accelerated the speed of land communication, Darius I still believed that waterways must be used to establish connections between all ends of the empire. He sent Silax, a Carian sailor, from the easternmost province of the empire to follow the water route to the nearest Indus valley, and he was ordered to cross the Indus and the Indian Ocean to reach the Red Sea coast of Egypt. When Syracuse had accomplished his mission unerringly, Darius annexed the Indus region. Either before or after this, he completed the canal from the easternmost branch of the Nile Delta to the Gulf of Suez, which had been begun by Pharaoh Necho II. Xerxes wanted to repeat Necho II's great voyage around Africa, but his fleet set sail from the Mediterranean instead of the Red Sea. The voyage ended in failure. The maritime ambitions of Darius and Xerxes were not inherited by their successors.
Post roads were established across the country to facilitate the mobilization of troops, the issuance of king's orders and the communication of subordinates' information. Strengthen connections across the empire. There are stations along the route, equipped with personnel, horses, and food. The longest post road starts from Ephesus on the west coast of Asia Minor and ends at Susa on the Iranian Plateau in the west. It is 2,400 kilometers long and is called the "Royal Road". Another famous post route was from Babylon to the Indus River via the Iranian Plateau. This kind of post road may have been built on the basis of the original large caravan roads and some post roads during the Assyrian Empire.