There several references to "Gran Hotel" including a character from Cantaloa who's mother works at the hotel.
Netflix had originally ordered a fourth season, but after production was going to start, it was decided that the season wouldn't happen and that the show's ending would be with season 3.
In early episodes as Clara sings with the ship's orchestra her microphone is angled toward her so she is singing into the solid top. The microphone used is a dynamic microphone popular in the 1940' s. To work correctly the microphone should be angled away from her or straight up (as it is in S1 E8) so that she is singing into the sound slots on the side.
Black nail polish was introduced in the 1920s and therefore is appropriate for the era and added mystic to the character of Cassandra as a spiritual medium and tarot card reader.
Season 3 Dr Ayala mentions his father who was an Inspector. This is reference to the Inspector Ayala who was also played by Pep Anton Muñoz on Gran Hotel.