This supernatural fantasy series heavily features the number 4 and associates events with the number 4 to the hotel (take the Subway Line 4, hotel room number 404, 4th floor of the hotel). In South Korea and some Asian countries, the numerical digit 4 is considered an unlucky number and is associated with death (due to the words 4 and death sounding similar in their native languages). The fear of the number 4 is known as tetraphobia and is taken seriously in some Asian countries that sometimes floor levels or rooms with 4 do not exist in hotels and other buildings. Also mentioned is the specific time period of "49 days", which is an existing belief that deals with death and the afterlife in some religions and philosophies such as Buddhism, Shamanism, Taoism, and Bardo.
The hotel location was filmed at the Mokpo Modern History Museum No.1