The temporary exhibition, titled The Secret Chambers of the People's Authorities, takes visitors into rooms designed for communist elites during the castle's reconstruction in the 1970s.
The title refers to the country's official name under communist rule, the Polish People's Republic.
Located in the castle's southern wing, the rooms have been used as offices since the 1980s. They were originally planned for official functions and as guest accommodations for foreign dignitaries from other communist countries.
Photos: PAP/Radek Pietruszka
The project began in 1974, after the basic reconstruction of the castle had been completed. It included extensive refurbishment and the construction of a banquet hall, a conference room with interpreters' booths, kitchen facilities, air conditioning and a lift shaft.
The lift, however, was never used.
Visitors can enter the shaft and listen to testimonies about various aspects of life under communist rule, which ended in Poland in 1989.
Asked by state news agency PAP which part of the secret chambers made the biggest impression on her, Royal Castle director Małgorzata Omilanowska pointed to the bathrooms.
Małgorzata Omilanowska, director of Warsaw's Royal Castle. Photo: PAP/Radek Pietruszka
"Decorated with white Carrara marble and black limestone, and equipped with Western fittings, they look extremely pleasant even today," she said.
Omilanowska said the idea of using the Royal Castle as a residence for foreign dignitaries from fellow communist countries was reportedly favoured by communist party leader Edward Gierek.
Neither Gierek nor any of his successors used the castle for that purpose, however, because of opposition from prominent historian Aleksander Gieysztor, who was then director of the Royal Castle, and art historian Stanisław Lorentz, who was director of the National Museum at the time.
The Secret Chambers exhibition runs until January 17, after which the rooms will return to use as offices for Royal Castle staff.
(mk/gs)