How did Darius treat those he conquered?

Answer 1

The purpose of the Persians was to create a peaceful, secure and prosperous empire. When they took over areas, they would leave local rulers to continue to control the city/tribe/people and establish a Persian provincial governor to oversee them, and also keep the peace and defend against external intruders.

They would also impose a tax to finance this defence and to undertake public works. It is interesting that when the european Greeks liberated the Greek cities of western Asia Minor, they had to settle on tax systems for the cities. The Athenian general Aristeides, who was appointed to do this, ended up using the Persian tax model, which he found to be very fair.