This is an old postcard of the Castle San Angelo in Rome. It is an undivided back card, which mean it was made in the very early years of the 20th century - probably before 1905. This is an Italian card - all the captions and other information printed on it is in Italian, except for one French translation telling you that this is indeed, an Italian postcard.
I'm pretty sure the caption translates "Rome - Bridge and Castle San Angelo". I've actually been there, and it still looks very much like this picture, except it's in color.
This structure was originally built about 1900 years ago as a tomb for Emperors of Rome, and was later fortified and used as a safe haven for Popes. Its not far from St. Peters, and I believe there is a tunnel leading from the Vatican to the Castle. When it was built, I'm pretty sure they didn't call it Castle San Angelo.