The Black Teeth is the name for the ancient fortress of the Machine Men, located just north of the Trench's intersection with the Hornspine, in northern Acirca. It has existed since the Green Age, and served to protect the dwindling population of Machines from the still-vengeful warlords who followed the Machine Dominion. On its gates are written the words "Omo's Spawn" in Machine-Script, which is in reference to Omo, son of Omem, whose curse caused the Machine Men to live only centuries.
The fortress of the Black Teeth is at its center composed of four flattened areas, which surround the Grand Watchtower at each cardinal direction. To the south is the Entranceway, which has areas for public discussion. Its southern boundary is marked by the gigantic stone gates of the Teeth. To the north is an area reserved for meetings, usually military, as this area also includes the secret "Forge" of the Machine Men. To the west is the Library, which is considered the best in Acirca but largely kept closed to non-Machines. To the east is the garden, which contains a Tree of Thought<ref>Rice Boy (First Edition), Map of the Black Teeth</ref> and is the ultimate source of the Sunk River. It also contains the Judging Stone, which was used as a sort of courtroom by the Judicials.<ref>Order of Tales (Book II), Appendix: "The Black Teeth", p278-9</ref>