Venicela serenissima

The 14th-century Palazzo Ducale, or Doge's Palace, housed the Venice's senate, hall of justice and private apartments of the Doge until the fall of the Venetian Republic in 1797. General admission lets you peruse the Doge's apartments, ornate assembly halls of the former ruling nobility and prisons on the other side of the diminutive Bridge of Sighs. But for a few euros more, you can join a guided "Secret Itineraries" tour that takes you into a warren of rooms under the palace roof. It includes the secret state archives (an impressive room designed like a ship captain's quarters) and the jail cell where that old playboy Casanova was incarcerated before he escaped.