The build a world thread

[Caput]

In this thread, we build a world. Each post adds a new element to the world, or expands on an old one. You can add anything you like, even to things other people have invented, regardless of tone, genera or size, as long as you aren't fucking it up on purpose or contradicting something that's already been said.
This world doesn't have to be designed for anything in particular and anybody who contributes can use it for whatever they like.
I'll start with this:


The Wandering Citadels

The Citadels are castle-cities built on and carved into the shells of tortoises. Each is armed with 4-8 giant cannon and carries several smaller beasts and vehicles, as well as large stores, homes for all it's inhabitants a number of surprises for attackers.
The great beasts have a symbiotic relationship with the people who live on their backs. The people scout ahead and find food and water, turn their great guns on the larger animals who eat tortoises, tend to their wounds, guard soft shelled babies and trade resources and information with other people for mutual benefit.
Wanderers frequently battle over the sparse resources of their homeland, but for the most part try not to kill each other's tortoises, as they are valuable and respected.
Citadels periodically visit static settlements near the edge of the wastelands, trading their feared weapons and things from the opposite side of their homeland for food and other mundane goods that are hard for them to get. The wanderer culture and those cultures they trade with are alien to each other, but everyone celebrates when a tortoise comes to town, because the wanderers are honourable and there is much money to be made.