On Thursday I got thirteen postcards including three from Monaco.
One of the Monegasque cards shows the Prince's Palace. The Prince's Palace is since 1297 the home of the Grimaldi family. Unlike other European palaces it did not become a luxurious palace, but became more fortified. Today it is still the official residence of the Prince of Monaco.
With a matching stamp:
800 years Fortress (issued 11-07-2015)
Thank You very much William!