The Vatican City has a number of unique claims to fame, but one of the lesser known facts is the way in which it is able to guard itself. You see, this is the only country in the world that is able to protect itself at night by simply closing and locking the gates. In doing so, the entire country, well there are just over 107 acres in the country, is sealed off from the rest of the world.
Of course, you can imagine that it would be extremely difficult for any other country to be able to do this due to the sheer scale and size of the wall and gates that would be required. This is obviously something that has been left alone for centuries, but even though they do not actively go ahead and close the gates in this way, it still remains an option.