Every adventuring group needs a place to fence plundered goods and heal grievous wounds sustained during adventures. For the Spelljammer setting, a setting where players sail wooden ships through the fantasy equivalent of outer space, the Rock of Bral is the best they can hope for.
I have to confess that back in the day, my fourteen year old brain only scratched the surface of what the Spelljammer setting had to offer. It quickly devolved into Space Dungeon at times, using the setting as a way to have dungeons in asteroids. However, the Rock of Bral was something I quickly wrapped my head around.
The Rock of Bral is one of the most detailed fantasy settings in all of Dungeons and Dragons geekdom. The setting is detailed from it's early days as a mind flayer outpost, to a pirate headquarters, to a thriving fantasy city catering to all the needs in wildspace.
Due to the fantasy physics of the setting, both the topside and underside of the city are detailed. While the underside is little more than a prison, the topside is fleshed out to a fantastic degree, detailing the noble houses, the neighborhoods, the underbarons, and much more, giving the Dungeon Master everything he needs to run a long campaign without the PCs ever leaving the confines of the city.
For running a Spelljammer campaign, two resources are essential: the Spelljammer: Advanced Dungeons and Dragons Adventures in Space and this book.