In the late ancient period of the Roman Empire, Constantine ("the Great," as he is often labeled) was tremendously important for several reasons. For one thing, he brought much-needed stability to Rome after years of civil strife. For another thing, his adoption of the Christian faith brought Christianity into the center of Roman civilization and thus into the center of European life as a whole -- even long after Rome itself was no longer influential in Europe.
Constantine the Great did not do anything to the capital of Rome. He create a new capital for the Roman Empire. He designated Byzantium as his imperial capital, redeveloped, renamed it after himself (Constantinople, which means City of Constantine) and inaugurated it in 300.
I guess that by Constantine you mean Constantine the Great. Constantine and Ambrose were not really contemporaries. Ambrose was only 7 when Constantine died. He was not involved in decisions regarding the empire. Ambrose is better known as St Ambrose. He was the bishop of Milan and one of the Latin doctors of the Church, or 'fathers' of the Catholic Church. His theology contributed to laying the foundation of the Catholic doctrine. In payed an important part in shaping the doctrine of the immaculate conception. He saw virginity as superior to marriage and Mary as a model for this. His writings on ethics were also important. He also opposed the Arianism of emperor Velentinian II.
The arch of Constantine is in the city of Rome, next to the Colosseum.
Constantine was the Roman emperor from 306 or 307 to his death in 337.Constantine was the Roman emperor from 306 or 307 to his death in 337.Constantine was the Roman emperor from 306 or 307 to his death in 337.Constantine was the Roman emperor from 306 or 307 to his death in 337.Constantine was the Roman emperor from 306 or 307 to his death in 337.Constantine was the Roman emperor from 306 or 307 to his death in 337.Constantine was the Roman emperor from 306 or 307 to his death in 337.Constantine was the Roman emperor from 306 or 307 to his death in 337.Constantine was the Roman emperor from 306 or 307 to his death in 337.
Constantine I (or the Great) was the 57th Roman emperor.