Pharaoh Ramses XIII is a fictional character. While the name Ramses/Ramesses was relatively common for Pharaohs of the 19th and and 20th Dynasties, there were only 11 historical Pharaohs of that name. Two of them were members of the 19th dynasty, and nine of them were members of the 20th dynasty.
The film is based on the historical novel "Pharaoh" (1897) by Boleslaw Prus. The setting of the novel is 11th-century BC Egypt, towards the end of both the 20th Dynasty and the New Kingdom. It was a time of decline for Egypt.
Among the consultants hired for the film was archaeologist Kazimierz Michalowski. He was an expert on Egyptology and was at the time involved with a committee trying to rescue the Abu Simbel temples from destruction.
While Boleslaw Prus, the author of "Pharaoh", wrote only a few novels, he is considered one of the best novelists of his times. Fellow writer Joseph Conrad was a fan of his work and considered him a better writer than Charles Dickens.
For the needs of the film, an artificial island was created on Lake Kirsajty in Poland. Real palms and lotuses were planted on the island to represent Egyptian vegetation.