We took the tour that included the map room's secret passageway to Bianca’s eavesdropping chamber. Bianca was the mistress, and later the wife, of Grand Duke Francesco de’ Medici. He respected her opinion so much that he built her an eavesdropping chamber over the Salone dei Cinquecento, where important government matters were discussed. The peep hole is disguised as an air vent and gives you a great bird's eye view of the salon.
There are other tours of the patrol path (the crenulated area around the top) and also a "secret passages" tour. They also have workshops for kids and actors in period costumes. Check it out here.