The massive Venetian walls encircling Città Alta were built starting in 1561 and took decades to complete. The main gate, Porta Sant'Agostino (1575), is the grand entrance at the top of Viale Vittorio Emanuele.
You can walk the top of the walls — there are parks, benches, and views stretching for kilometers over the lower town and the Po Valley beyond.
A story
Venice spent the equivalent of an entire year's state revenue constructing these walls between 1561 and 1590 — a defensive investment triggered by the Ottoman threat to the eastern Mediterranean. The irony is that the walls were never tested in battle. By the time they were finished, the age of siege warfare they were built to resist was already passing.