What are some military accomplishments or advancements of the Persian Empire?

Persia and its associate Media expanded the Empire to stretch from today's Libya in the west to Pakistan in the east. It held this empire together for two centuries by maintaining internal and external security by mobilising local forces, reinforced by Persian-Median central forces. It also maintained a strong naval force even though it had no maritime forces by mobilising the navies of Phoenecia, Egypt and the Greek city-states within its borders.