The Guardian is an enchanted stone head found in Constantinople that asks three questions of those visiting its underground cistern [143]. Professor Childermass, Johnny, and Fergie must answer its questions or face certain death. It identifies itself as being "set up in the time of the great Emperor Justinian" and being able to speak all languages on earth - thankfully English. Once completing their quiz successfully, the three adventurers escape and see "in a niche near the door stood an enormous stone head. With blank eyeballs it stared eerily out at them" [146-7].

The Basilica Cistern, or Yerebatan Cistern, is a vast underground reservoir located beneath the northern end of the Hippodrome. Built in 532 AD by Constantine the Great, it is the largest surviving Byzantine cistern in Istanbul. This sunken palace is supported by 336 columns pillaged from classical buildings during its construction; two columns used to support the roof are themselves supported by carved Medusa heads. One head is on its side, the other inverted - positions that suggest the people who put them there were Christians and did not want to revere a god of a pagan period. It makes you wonder if these provided Bellairs with some inspiration for his own Guardian of the Sunken Palace. Apparently, it was a largely forgotten subterranean ruin and only rediscovered again in the 19th century when the authorities noticed that locals were collecting water and fish by lowering buckets through holes in their basements. Today, water eerily trickles down from the ceiling above into the pools and planks allow visitors easy access among the decorative columns. You may also hear classical music piped in through speakers and live concerts have been held there as well.