It's never too late to satiate your desire for World Domination in M&B:W. Well, that's how I'd handle the situation. Google for it and it will explain a lot of this. It would be advisable to capture said castle after the majority of the Kahnid noyans and their Khan have just recently fought a major battle, and are low on troops. Lastly - There is a guide out there for how to be an effective Marshall in Warband. Get some lords on your side, preferably Kahnid, to support your cause, and make a stand at the castle should the Kahnid come at you in numbers. Be careful when assaulting the castle though, as the Kahnid are very archer-oriented, making the ascent on top of the castle walls a rather interesting experience.

This castle is in a narrow passage, and should make it easy enough to defend or flee from if necessary. Once you have about 20 mamlukes, the Sarranid melee horsemen, (gonna take some time, but given that Swadia's gone (they're always first to kick the bucket) and you're cutting into Vaegir and Sarranid territory, 20 should do it), defect and try to conquer their southern-most castle (forgotten its name). Most strategy games have you working as some. Make up for the lack of ranged units with some horse archers of your own though. New Game Strategy Sleeper Hit Mount & Blade: Warband Comes To Android As A Tegra 4 Exclusive.

They are excellent detergents towards Kahnid Horse Archers, because of their extreme resilience and armored horses. I suggest you build an army of Saranid Mamlukes.

Mount and blade warband strategy