@reason Crab settings are good, as I think was mentioned somewhere I used a Crab city for my game, and I thought it was an element that worked well. Being close to the Shadowlands adds options for more traditional 'adventuring.' And although L5R is notorious for inconsistent maps, the Crane, Mantis (or their Minor Clan predecessors), Scorpion and Unicorn are all vaguely close by so you have those interactions to work with in terms of big politics.
You'd probably want to expand to the castle town vs. strictly focusing on the actual fortification. Even if it gets more outsiders than others, the garrison would be overwhelmingly Crab and pretty regimented in terms of their daily lives. The town would let you build on that with whatever local traders, entertainment, local temples, etc. to fill out a more lively and mixed population. You would have to do it yourself, as I don't think there's any kind of write-up for it, although I wouldn't swear to it with all the material L5R has. Regardless, it is kind of an unlikely setting for 'high' politics, as it wouldn't qualify to host an Imperial Winter Court. However, lower level courts also exist. I think this is kind of a hard question to answer in a vacuum without getting into the full scope of your setting, including things like time period and what conflicts you want to focus on as overarching stories.
Basically, there's waaaay too much material to easily summarize, so it really depends what you'd be most interested in. If you just want to hunt for ideas, I recommend, again, the Emerald Empire (any edition) and both Imperial Histories (4e) books, as well as the older Way of/Secrets of the Crab (1e and 3e). FFG has Courts of Stone which is both castle and court-focused. Strongholds of the Empire (4e) is where I got some of my Sunda Mizu Mura info (although it appears a couple places), and while it doesn't have any other Crab locations it might be worth a glance just to see the way they tend to present setting info.