Monstro City is the underwater home of a race or group of races that from a mundane person's perspective, would be best described as intelligent monsters. The city is located somewhere within the area of the Bermuda Triangle and the Atlantic Plains. Furst Family team member, Rex Zorus describes it as "a jewel in the ocean depths."
Madame Majestrix is the reigning queen and ruler of the city and has been for many decades. Her son, Prince Rex Zorus, and his daughter, Princess Astra Jeannine Majestros-Furst, are second and third in line to the throne, respectively.
In the mid 1990s, the city was in a demolished and abandoned state. Nearly all the inhabitants vanished -- the result of a war with an underwater society known as the Trenchers. It's unknown exactly when this battle took place, but probably relatively close to the time the Furst Family visited, while searching for their missing teammate, Astra Furst. Speculation runs high that the city has been rebuilt and is once again a thriving metropolis.
Long ago, Monstro City's citizens established an extensive base on the moon. In 1984, a member of that colony awoke from a death slumber, and in a frenzied mental-state, attempted to crash the moon into the Earth. This effort was thwarted by the First Family alongside Madame Majestrix.
Once, there were millions of residents within the confines of the city proper. There is no information provided on the number of "Monstroids" that have returned, or how many are still in relocation areas.
Monstro City's citizens vary widely in form, color, size, appendages, organs, and other bodily parts and pieces. Looking not so much like one species within a very narrow range of DNA encoding, but instead, almost like multiple species, with a very broad and dynamic DNA "footprint." They are still very much a part of the hominid branch, but they seem to stretch the genetic extremes to their limits. They (some? most? all?) can (and do) produce offspring from homo sapient relationships. Unless there is something that the creative team is holding back about their origins, for all intents and purposes they are human.
To date, no published Astro City stories have studied their ancestry and DNA makeup in depth.
Judging by his title, Bertos Grimlar Seneschal, the naming conventions of Monstro City seems to be patterned with the surname as a designation of one's profession.
Madame Majestrix's army is known as Monster Troops by the rest of the world.
While most or all of its inhabitants cannot breathe underwater, the city is surrounded by the Atmo-Sphere, a bubble-shaped forcefield which holds back the watery atmosphere of the ocean