Why go
The ancient city is less visible than the towers and churches, but it explains Bologna's long role as a crossroads. Look at the alignments of central streets, then use the Museo Civico Archeologico if you want objects and a fuller account of the Etruscan and Roman periods.
The full story
The settlement known to the Etruscans as Felsina developed centuries before the medieval city visible today. In the fourth century BCE, the Celtic Boii took control; Rome later conquered the area and established the colony of Bononia in 189 BCE. The Via Emilia, built soon after, helped turn the city into a link across the Po Plain. Roman planners set Bononia around crossing main streets, with smaller streets dividing it into blocks. Later construction changed the fabric, but the old grid still gives the centre some of its underlying order. This is useful context when you move between Piazza Maggiore and the eastern streets: the city did not begin with its famous medieval towers. If you want to see material evidence rather than infer the layers from a map, the Museo Civico Archeologico provides a more focused visit. Its collections also place Bologna's Etruscan past alongside the later Roman city.