Hy-Avalonia is the capital of Avalonia, it is also the Mothership that the Astralus traveled on to Aquatica. The city can't be entered from below, it floats in the sky as does Hy-Altia.
Hy-Avalonia is a World Tree as well as the Mother Ship of the Astralus. She is a Magitech creation, a combination of a world tree and spaceship.
The ship is the core of the floating city of Ynys Avalon, which surrounds the mothership. The city has been added to over the centuries and is now overshadowing the countryside below it.
Astralus, Elves, and various others.
Hy-Avalonia has many defenses, one of the many is the Veil, its ability to turn etherial and invisible.
The city has all the modern conveniences, they are discretely hidden away, and only seen when needed.
The city is split into various sections, the lowest section is maintenance, and the highest is for the ruling class and nobles. There are numerous levels to the ship, the central axis is open throughout its height. The upper levels are for the upper class, although those living lower can earn their way higher by the merit of their work.
Most of the machinery of the ship is built-in and can't be easily removed. There are many storerooms, and others for education, recreation, work, and for many other uses.