Along with many, many other uses, Venice's famous Doge's Palace once had a few corridors serving as prisons. The original palace prison earned the nickname “Piombi,” Italian for "lead," because it was in the attics underneath a lead-covered roof.
A lead roof was undoubtedly put in place at least partly with the intention of making the prison impenetrable, and this is what it became known for. The Piombi also had a reputation of being notoriously uncomfortable; it was extremely hot in summer and very cold in winter, making it an even more undesirable place to be incarcerated.
Perhaps the most famous resident of the Piombi was Giacomo Casanova, who successfully escaped from the supposedly inescapable prison in 1755.
The Piombi attic prison is generally closed to the public but is featured on a "Secret Itineraries" tour of Doge's Palace, along with the torture chamber, ducal administrative offices and the Bridge of Sighs.