Umbriorr has only three doors, set at a perfectly equal distance apart. These doors are kept locked, even from members of the legion. They are the only ways in and out of the city, and are not used.
Umbriorr was built in Ortis, 'the Birth Year' when Nect came to power and fought off the attacking Indocti. It was built under Nect's instruction and was used to keep away the invading barbarians, a job it completed successfully.
Following the 213 Arbitor Uprising and the creation of the Legion of Nect, that force was assigned to protect the wall.