I remember seeing this film on television multiple times in the early 1960s. It is roughly in the same style and genre of another Jules Verne story and film, "In Search of the Castaways" (1962), which was made into a film at almost the same time and was also shown multiple times on television around 1962-1964. As children, my sister and I were captivated by both these films, and we loved to re-watch both of them on television in those years whenever they were shown.
By today's standards the effects and acting are weak, but the film is still captivating, memorable, and fires the imagination. The film has a lot of particularly memorable imagery: a basket balloon, a castle-like fortress on a beach, chained gorillas in a dungeon, the lost treasure of Cleopatra, a very tall and eerie black man who can't speak because his tongue was torn out, a torture chamber with a stretching rack, a woman threatened with torture, a man jumping into a lake from the basket balloon, being chased by African natives with spears, cannibals, a shrunken head hanging on a rack, a fight with a medieval mace, a condor attack, having to dump priceless treasure overboard, and so on. Some scenes border on film mastery. For example, it is surprising how much tension could be put into a simple scene of a man gathering water at a river to take back to the balloon while the audience knows that cannibals are watching him from behind the coconut palms. The aerial photography is reasonably inspiring, although admittedly the filmed backdrop effects aren't high quality.
To clear up some common misperceptions: the basket balloon in the plot is actually a hydrogen balloon, not a hot air balloon. In the story, acid and water are carried on board to generate hydrogen, which is obviously dangerous and this fact figures into the plot since it is too dangerous for the balloon's occupants to fire the musket that they brought along, which leaves them largely at the mercy of attacking condors, spear-throwing African cannibals, and an unwanted guide. Also, the date of Friday, April 7, 1878 is shown at the beginning of this film, on the sign outside the lecture hall, so this is definitely not the medieval period.
The plot is solid and the imagery great. If only the acting and effects were improved, this could be an outstanding film.
