Short answers, pulled from the story.
The Celtic tribe known as the Insubres established a settlement called Medhelanon in 590 BC on the fertile plains of northern Italy. This community belonged to the Golasecca culture and thrived near the confluence of rivers that would later define Milan's geography.
Emperor Diocletian relocated the Western Empire capital from Rome to Mediolanum in 286 AD leaving his colleague Maximian to govern from this northern stronghold. The city grew into Cisalpine Gaul establishing trade routes connecting Mediterranean ports to Alpine passes while population numbers surged from 40,000 residents during Augustan times to approximately 100,000 people under imperial rule.
True devastation arrived during the Gothic War when Uraia laid waste to Mediolanum in 538 according to Procopius who recorded losses reaching 300,000 men during this catastrophic conflict. The Lombards captured Ticinum renaming it Papia making it their capital instead allowing Early Medieval Milan governance by archbishops alone after Roman control ended permanently.
Emperor Constantine issued the Edict of Milan in AD 313 from this northern capital granting religious tolerance across the entire empire. Christian communities had already existed within Mediolanum contributing martyrs during periods of persecution including Saints Gervasius Protasius Victor Maurus Nabor Felix Nazarius Celsus and Sebastian before Merocles became the first bishop with firm historical presence attending the Council of Rome in 313.
A tower measuring 16.6 meters high remains inside the Convento di San Maurizio Maggiore representing part of the original circus structure while Moenia walls and twenty-four-sided towers survive at various locations including piazza Missori and beneath modern metro stations. Ruins of the imperial palace lie scattered near current urban development while Baths of Hercules remnants appear in the Civic Archaeological Museum alongside Colonne di San Lorenzo standing before the church of San Lorenzo displaying four surviving columns from ancient times.