He gave an end to the Byzantine Empire by capturing Constantinople and other Byzantine cities left in Anatolia and the Balkans. The invasion of Constantinople and some more successful campaigns against various small kingdoms in the Balkans and some other Turkic territories in Anatolia bestowed immense glory and prestige on the country and the Ottoman State was started to be recognized as an empire for the first time.
As can be guessed from his successful campaign against Otranto in southern Italy and his adopting the title Roman Caesar, he was presumably trying to vitalize the Eastern Roman Empire. For a probably similar reason, he gathered Italian humanists and Greek scholars at his court, kept the Orthodox Byzantine Church functioning, ordered the patriarch to translate the Christian faith into Turkish and called Gentile Bellini from Venice to paint his portrait.
He is also recognized as the first sultan to codify the criminal and constitutional laws long before Suleiman the Magnificient (Lawmaker) and thus establishing the classical image of the autocratic Ottoman sultan (padisah). After the fall of Constantinople, he founded many universities and colleges in the city, some of which are still active.
see Ottoman Emperor