The Addizione Erculea — the Herculean Addition, named for Duke Ercole I d'Este — is the Renaissance urban expansion that earned Ferrara its place on the UNESCO World Heritage list. In the 1490s, the architect Biagio Rossetti laid out a new quarter north of the medieval city on a rational grid of wide, straight streets terminating at deliberate visual endpoints. The result is still visible and still walkable: Corso Ercole I d'Este runs north from the castle to the city walls with an architectural grandeur that surprises visitors expecting a typical Italian medieval town.
The two great palaces at the top of the Addizione — the Palazzo dei Diamanti and the Palazzo Prosperi-Sacrati — face each other across Via Ercole I d'Este and demonstrate the ambition of the project. Walking this street and continuing to the Montagnone garden and the city walls gives you the clearest sense of what 15th-century Ferrara was attempting: a rational, beautiful, humanist city from scratch.