Have you checked all the statues using the Statue Telepathic Detector?
Have you followed the statue's clue to get the grey key?
Jump to Piazzetta. Turn around while looking down at the Piazzetta. Go forward, climb the steps of the building and turn right.
See another Gravitiello's invention - the sparkling screen-barrier. Based on the statues' clues, use the grey key on the keyhole at left. The barrier is removed.