Leviathan is Aizen and he seems to have brought his gambit planning skills with him.
The country of Sauville really existed. In the real world, it was known as Savoy. It became an independent Duchy somewhere during the Middle Ages and existed as a country until it was annexed by France in 1860 during the Italian unification movement. It was mostly located on the slopes of the Alps (hence the mostly mountainous landscape in the series), but also had control of the city of Nice, which was a port city. While, of course, the history and the place names of Sauville are made up, the geographical location and the landscapes are fairly accurate.