Both the Greuthungi and Thervingi became heavily Romanized during the 4th century. This came about through trade with the Romans, as well as through Gothic membership of a military covenant, which was based in Byzantium and involved pledges of military assistance. Reportedly, 40,000 Goths were brought by Constantine to defend Constantinople in his later reign, and the Palace Guard was thereafter mostly composed of Germanic warriors, as Roman soldiers by this time had largely lost military value. The Goths increasingly became soldiers in the Roman armies in the 4th century  leading to a significant Germanization of the Roman Army. Without the recruitment of Germanic warriors in the Roman Army, the Roman Empire would not have survived for as long as it did. Goths who gained prominent positions in the Roman military include Gainas, Tribigild, Fravitta and Aspar. Mardonius, a Gothic eunuch, was the childhood tutor and later adviser of Roman emperor Julian, on whom he had an immense influence.