In any case the worship of Aphrodite was more prominent and the catacombs were connected to the Temple of Aphrodite nearby. They housed a statue of the goddess, the walls were adorned by pentangles, spirals, murals of Adonis, Hippolytus and Pygmalion, and a cursed priest of Aphrodite was banished in a sarcophagus there.
Over the years they became a dangerous place populated by poisoned rats. They were sealed and only the Hierophant could open the way to them from the Street of David, giving to the worthy ones an elixir to survive. However Galahad who gained access to them was stranded there and left in a coma.
Arthur gave Galahad his own elixir and survived thanks to a rejuvenating apple given to him by Tamra. Then after giving life to Aphrodite's statue, he answered six questions about the cult of Aphrodite. Then it gave him instructions to find his way through a labyrinth of doorways to the Temple outside.
According to the geography of the Old City of Jerusalem, the area between the Street of David and the formed Temple of Aphrodite is Muristan. Although it's not mentioned in the game, the catacombs supposedly lay below that area.
It's possible to escape the catacombs before or without meeting Galahad, however the Grail will punish Arthur.