How did Persian kings maintain control and organize such a vast empire?

The
Persian empire was able to control such a vast and expansive area mainly due to their combined
system of centralized and decentralized hierarchical control. The Persian empire held
centralized power through the king who ruled over the entire empire backed by the military
strength of the enormous Persian military. The empire was sectioned off into regions known as
"satrapies". The satrapies were ruled by local rulers (sometimes native to the
conquered land) called "satraps". These satraps were under direct control of the king
but were able to focus specifically on ruling their designated regions. While the Persian empire
allowed for some religious and other freedoms, the empire still maintained its absolute control
through the fierceness and size of its army.