The Tombs of the Ancient Kings were a series of structures on the southern edge of the Great Desert, just beyond the Calormene capital city of Tashbaan. As their name implies, they were likely the burial places of past rulers (whether of Calormen or some other nation or culture is unclear).
There were about a dozen tombs in this location, clustered together in a random formation. They were described as resembling enormous stone beehives, but narrower in shape, and each structure had an opening at its base. According to Calormene fables, they were supposedly haunted by ghouls.
Shasta, Bree, Aravis and Hwin met there after going through Tashbaan. Shasta slept there for a night unharmed, though the myth led him to be frightened by a cat (which was actually Aslan) during the night while he was there.