Re: Tell me about your campaign world.
I ran a campaign for 30 years in 4 cities -- Ryoku the Game of Power. It dates back to 1100 AD Japan. The Emperor banned it as too violent, and was killed soon after. It was kind of a chess game with 8 sides. Each suit (steel, urmetals, crystal, souls, shadow, fire, plus Justice and Chaos) had a base set of powers and disads. Characters or villains were often "pieces" (about 30% of the time) and eventually they start to collect crowns. It can tie in a wide range of character types (martial arts, magic, machine) and give them a common conspiracy world to skulk around in, as well as adjacent dimensions.
One character accidentally becomes the Palladin of Justice -- the target of assassins.
I eventually wrote a book about it. (Foundation for the Lost, on Amazon and Smashwords in electronic or paperback format)