How to get a Castle, or a Town:
Start with a highest possible steward perk you can get and max it out. easy
Made a trader live in the meantime and rumble around to get some money on your hand. easy
Search for nobles in villages and try to improve relation. easy but annoying
train and hold battanian fians. medium (because at that point you wont want auto resolve and lose some. also you need passion)
Once trading works out for you, you should have a good eye on the map and see some week spots like huzn fulq (gesundheit), or tyal. Target should have equal or less troops in it, you have. easy
Wait for an open attack window and make some hostile action on the faction, that hold the city. medium
attack that city! But do not waste time to try to break some walls. if you feel comfortable you can try to build a ramm, but i will suggest it is not worth it.
If you play an archer (because why not? You have fians!) you can snipe a lot of troops from the wall. run foward and grab a ladder. troops will come and help you and then make an charge. sometimes the enemy troops open the door herself and run out to you.
If you play a infantri: LOL Why not, go ahead.
So now the hard part, creating a kingdom.
If you do the mainquest leave almost all troops in your new home because it buys you time. The enemy hate you now a little and try to get that back. if you lucky there try to get you first. but you should be faster now. if you are not lucky you have to run faster to the questgiver you choose.
If you choose to be Clan tier 4 and create without help, you will find yourself in the very hard spot. because you need at least 100 influence to declare peace and that can be a little tricky to reach.
(tl;dr) I´m old, english is hard!