Right now I have 2 castles and 1 cities, but I left after I got them and build my own kingdom. But the way I took 3rd is bit tricky tho. It doesn't appear on me also but I used my influence points to change politics of kingdom to make myself earn more and more influence, things like + influence on castle owners, + influence for each notables in fiefs, + influence for each fief etc etc. then I had huge amount of influence just as others in the kingdom which makes influence soo less in Kingdom but after that, I offered my target castle to rechose owner and it takes default influence which is nothing when you had 2.6K influence...
Monchug had around 7-10K tho
but I used all my influence on castle to gave it to me even with that, percentage was low like 37% but somehow I've chosen, probably because I have good relation with Monchug and target fief was belong to Khergits which is second strongest clan and most probably rival of king.