In Rome, some 170 private palaces, academies, homes and other buildings will open their doors to visitors.

This weekend marks the city’s second year of participating in the global Open House  initiative, founded in London in 1992. For art and architecture lovers, Open House Roma provides the rare chance to go inside everything from Rome’s first skyscraper to the ancient foundations of Hadrian’s Temple, not to mention to peek through locked doors and explore salons usually off-limits to the public – all for free.

Participating buildings include some of Rome’s finest architectural gems. A must-see is the Accademia di Spagna, the headquarters of the Academy of Spain. A former monastery on the Janiculum Hill, the site includes a round temple by Donato Bramante, considered to be one of the masterpieces of the High Renaissance, and 16th-century frescoes by Niccolò Circignani, nicknamed “Il Pomarancio”. Built in 1576 and reworked in the 17th Century by Baroque master Francesco Borromini, the Palazzo Falconieri, on the atmospheric Via Giulia in Rome’s historic centre (today the seat of the Academy of Hungary) is also worth a stop.

If you are a fan of the Neoclassical period, don’t miss the Casino Nobile di Villa Torlonia, a lovely villa in Rome’s northeast, designed by 19th-century architect Giuseppe Valadier and filled with works by famed Venetian sculptor Antonio Canova, his student Rinaldo Rinaldi and Romantic painter Francesco Podesti. Fascinated by Fascism? Check out the Palazzo della Civiltà in the EUR district, built for the never-held World Exposition of 1942, or the Palestra del Duce (literally, “Gym of the Duce”, meaning Mussolini) in the city’s northwest. And if you want a taste of something altogether non-Italian, your best bet is the Accademia d’Egitto at the edge of the Villa Borghese, which houses Rome’s first Egyptian museum.

Some government buildings are opening their doors, too. Visit the 15th-century Palazzo Madama near Piazza Navona, the carefully guarded seat of the Italian Senate, or the Palazzo Giustiniani, home to the President of the Senate’s apartment and where the Constitution of the Republic was signed in 1947.

Want to see Rome’s more modern side? Then head to Rome’s very first skyscraper, Eurosky Tower in EUR. Its full 120m height is still under construction (its 19th floor is open to visitors), but thanks to its 27 floors of luxury apartments and use of sustainable architecture, it just might be the most-talked about new building in Rome since the MAXXI, built by starchitect Zaha Hadid and opened in 2010. Or see how Rome layers ancient and contemporary at Galleria d’Arte 28, a contemporary art gallery located in the 17th-century Palazzo Cini-Ferrini, itself built into the 2nd-century Hadrian’s Temple.

Many museums across the city, including the Galleria Borghese, Centrale Montemartini, MACRO and MAXXI, are also participating, meaning they’re free to enter (although some must be booked in advance).

Although some buildings require advance reservations, most do not. Those that do can be booked online; if no spaces remain, visitors can still queue in the “rush line”, where they’ll be allowed to enter in place of no-shows.

Amanda Ruggeri is the Rome Localite for BBC Travel. She also writes