After the Romans took possession, Mediolanum became known for its schools and imperial buildings; it became the seat of a prefecture and eventually the Western Roman Empire. After Christianity became the dominant religion, Mediolanum spawned an important heresy.
Subsequently, the city was conquered (at least in our world) by various flavors of Goths, the Huns, the Longobards then the Franks. The Latin name of the city became worn down to "Milan" from frequent use. Various other empires took the city over due to its strategic location and productive lands, depending on who held the strongest hand at the time. Whether the Heterodynes ever penetrated that far south is a matter of sheer speculation at this point.
From the only map provided so far from the Guild of Shipwrights, Aeronauts and Cartographers, the southern side of the Alps down to the Po River is under the rule of Baron Klaus Wulfenbach, including the majority or all of Lombardy of the period; on the map provided to the right, Milan is roughly located near the "o" in the word "Kingdom" in Lombardy, and is included within Wulfenbach borders.