The Vatican

From the Sant'Angelo castle you can reach the Vatican by going through the same secret passage that the Popes used to escape in case of danger: the passetto di Borgo, that is, a long elevated 800mt long corridor, which connects the San Marco bastion of Sant'Angelo castle to the Vatican Palaces.
Usually it is not opet to the public but you could visit it through a guided tour.
It is highly reccomended to visit the Vatican even if you are not a religious person, because this, which is the smallest country in the world, contains treasures of inestimable beauty, an art heritage, history and culture, capable of thrilling Christians and others.
Starting from the “Cupolone”, the dome of San Pietro, whose view over the square is truly spectacular, to the Basilica di San Pietro, where, among other masterpieces, you can admire Michelangelo's Pietà, you can finish with the Vatican Museums, the Sistine Chapel, and the Vatican Gardens, and everything around here will thrill you. Quite a curiosity that not everyone is aware of about the Papal Guards: the recruits of this group must be Swiss citizens, between 19 and 30 years of age, and obviously Catholics, since they take a proper oath of fidelity towards the Pope himself.
This tradition dates back to the sack of Rome in 1527, when the Swiss Guards defended the Pope during his escape towards Sant'Angelo Castle.
Our advice in this case as well as the other places you must visit, is to book online the tickets or tu use a guided tour, in order to avoid queues and at the same time to get the most out of this experience.