The Tombs are a vast network of catacombs that sprawls beneath the castle and perhaps even parts of Sacor City. The age of them is unknown, but it is thought that, judging from ancient cave drawings of saber-cats that can be found, that the tombs have been in use for several millennia, by ancients called the Delvers, who dwelt in natural caves.
Over time, the caves were built upon age by age. Some time in the past, someone started burying the Sacoridian royalty and powerful objects of unknown, arcane origin down here. It has since become the resting place of all royalty, who are buried with the precious trappings they had with them in life- Queen Lyra was buried with her library of books, and King Smidhe Hillander with his favorite horses-- of which, it is unknown if the horses were dead or alive when buried. Upkeep of the dead is maintained by a village of "caretakers" that live within the Tombs. These are people that are caught down in the tombs and doomed to stay, or the more likely source, those who were born here. From time to time, they were moved above ground, but they find life under the sun hard to adjust to, as it seems to go against all they grew up with in taking care of the tomb's dead.
The layout of this tombs is a vast labyrinth with separate halls dedicated to many different royalty and the stretch of their families, which is the Avenue of Kings and Queens, and always ornately decorated with symbols and objects from the lives these Kings and Queens led. Chapels dedicated to common soldiers can be found, as well as a separate Heroes Avenue. Herein lies Sacoridia's great knights and heroes of old, the most notable of which is Liliedhe Ambriodhe lying with her great sword and wrapped in one of her tartans.
Because of the vast spread of the tunnels and precious nature of both the dead and artifacts that call the tombs home, the Black Shields have a specialized branch known as Tomb Guards that guard the preparation of the royal dead for their journey to the afterlife, and the upkeep of the bodies and artifacts from would-be intruders. It is this nature that probably prompts a strict law that only royalty and tomb guards may enter the Tombs. Anyone caught breaking this law is doomed forever to become a caretaker of the tomb. There are only two known clear exceptions to this. In Green Rider, King Zachary allowed several members of the cavalry and Karigan into the tombs via an ancient funeral procession entryway outside the city to get to his throne room and route Amilton's siege. The second, and far more peculiar, exception to this is one who masqueraded as of the Black Shields, as seen when Fastion allowed Karigan to wear their black as a sort of honorary Weapon when in pursuit of the book of Theanduris Silverwood. It doesn't seem likely that any negative repercussions will result from this perceived breach of law-- King Zachary was stunned at the occurrence, and the Weapons proved evasive when asked why they let her do it.