One big reason of the improvements of the Hakhamaneshian Empire, is that most kings were with merit. Most kings protected the empire and their citizens. They mostly care about the easement and welfare of the people, they didn't just think about their self and their power. Another reason is that, some kings like the Great Cyrus, expanded their kingdom without arguing and fighting. For example when Cyrus wanted to attack Babylon  he did it with out a fight and killing people. Because Cyrus told them if they let Babylon to be his, he would still let them to have their own emperor also he said that he promise that no harm will come to them. He freed all the prisoners and gave them food, water, clothes and a shelter. They come to a spot that both empires agree with each other. Last reason is that, kings were so knowledgeable and tried to learn lots of new stuff like: Math, Science, Art, Structures, Bridges, Roads, systems, statues  places etc. Also they created the very first money to trade for other stuff. This was really important because money is valuable, light and easy to use and carry, so thanks to them trading and buying stuff is much easier than before. They are many other reasons that helped Hakhamaneshian Empire to improve, expand their kingdoms and build a strong empire.