Don't be fooled by its simple exterior; stepping into St. John's Co-Cathedral is like entering a treasure chest of high Baroque art. You'll be surrounded by dazzling, gilded walls and an incredible floor, which is a tapestry of marble tombstones marking the graves of the Knights of St. John. The absolute must-see is Caravaggio's masterpiece, 'The Beheading of Saint John the Baptist,' a powerful and dramatic work that commands its own space. There's an entrance fee of 15€, but it's well worth it for one of the most stunning interiors in all of Europe.