It is estimated that in 480 BC, 50 million people lived in the Achaemenid Empire. The empire at its peak ruled over 44% of the world's population, the highest such figure for any empire in history. In Greek history, the Achaemenid Empire is considered as the antagonist of the Greek city states, for the emancipation of slaves including the Jewish exiles in Babylon, building infrastructures such as road and postal systems, and the use of an official language, the Imperial Aramaic, throughout its territories. The empire had a centralized, bureaucratic administration under the emperor, a large professional army, and civil services, inspiring similar developments in later empires. Furthermore, one of the Seven Wonders of the Ancient World, the Mausoleum at Halicarnassus, was built in the empire between 353 and 350 BC.