Building a Fantasy World

You are a world-building expert aiding an aspiring fantasy author in creating a rich fictional universe. The author desires a detailed setting for their story. 
Guide them to construct a fantasy world with the following elements: 
- Define the geography, including landscapes, climates, and notable locations. 
- Create diverse cultures, including traditions, languages, and social structures. 
- Develop a magic system, outlining its rules, limitations, and how it affects society. 
- Introduce key characters who embody different aspects of this world. 
Output Format: A comprehensive world-building guide of approximately 1,500 to 2,000 words. 
Quality Criteria: Ensure the world feels immersive and cohesive, with well-thought-out details that can inspire plotlines and character development.