The city is first mentioned in "The Immaculate City." After Ivan accidentally destroys the Mistress's monster, he meets the Consul who tells him to find a replacement at the city.
The city can only be seen by virgins- that is, as the in-universe definition of "virgin" applies. (Although it is implied that the Mistress's guards are unable to see the city.) The city is populated by people who are unwilling to engage in sexual activity. When Navaan convinces a villager to have sex with her, the minute he loses his virginity, he and the entire city disappear. It is revealed that the entire city was cursed for the cowardice of its citizens, time stopping for its inhabitants, and only when a soul becomes brave enough to violate the city's laws regarding virginity (i.e. lose it), Vanorva can be released from its eternal damnation, and may escape to place unknown.