possible but needs a slow build up. you can expand your party almost endlessly if you fight enough battles, winning tournaments, completing quests, ect. fill your inventory full of food and stock up on money. pull any soldiers you have defending a city/fort. leave any faction youre in and hit some really small faction that has limited territory. this could need 1-2 weeks of prep with another 2-4 weeks of actual combat and diplomacy/negotiations to get people to join you. gaining right to rule can help a lot.

or cheat, cheating is much faster but super boring

From what I've gathered really the only way to field enough troops early on is by saving up a ton of money and buying armies of Freelancers from the Freelancer guild and moving them to your castles and towns. This is really the only way to have enough manpower until you have a decent amount of vassals to actually be able to properly defend. You'll also have to fight a ton of defensive sieges at first.

引用自 GG LA Bunny Fuuu Fu：

When you start a kingdom by conquering a castle.
Umm...

the Cities have about 600 to 800 garrisons
which means u can never really conquer it alone.

so u need generals but you dont really have lands
to give generals either.

+ ur only castle gets attacked by some 1000 army.

My question is how is it possible to start your own kingdom
and take a city or unite Perisno??

The prime time to strike is when the Zann attack. Their units have either terrible shields or no shields at all. Their garrisons consist of around 1000 units. Do not fret though. Simply get an army of your faction's best archers (Aroulo Master Hurlers for the Aroulo) and when they sally out to defeat your army sit back, relax, and retreat before they can reach your archers. Rinse and repeat until they don't sally out anymore and you can get on with the siege. When you are finally prepared to siege them sit back with your archers and a handful of shieldmen and let the arrows fly. Retreat when your archers run out of arrows and siege them again. Rinse and repeat. Once you capture that pesky Zann town or castle IMMEDIATELY build a blacksmith and put all your archers and even more in the garrison, the assault on their lost land will begin soon. When the siege on your estate begins go afk because it will take a while for it to finish. Your archers will have a jolly good time killing off those Japanese wannabees, and thanks to the blacksmith you built they will have infinite ammo! Rinse and repeat every single step and conquer half of Perisno. Also, pro tip: in the Lord's Hall in Maccan there are priests, donate 1000 aurums to their cause and you will get 2 honor as well as positive relations with either the New Gods or the Old Gods, I suggest you get the relations with the New Gods in order to get the Holy Crusaders. I hope this helped, and be warned, the Aroulo Master Hurlers are the most OP unit in the entire game.

theyre also an ok place to attack since theyre so far away from anything and the factions leading to them are almost always at war with one of the other factions(at least in my games)

just be careful attacking their town as the units respawning just keep getting further and further off map x.x f'ing disaster when some units spawns off map and just sits there with no more ammo to deal with them.

not sure how easy a time others have dealing with them but my only issue is their spawn point moving

引用自 Sack Lunch：

I hope this helped, and be warned, the Aroulo Master Hurlers are the most OP unit in the entire game.

It depends on what faction you are, in one of my play-throughs, with the faction of 'Elintor' (master archers) I could quite easily capture Castles & Towns with a party size of around 180-190

Don't rush in like a crazy person, you will die - hold off outside castle walls within range of the walls, but not so close infantry can throw ranged weapons & let your archers absolutely smash the enemy. When ammo is running out & you notice enemy not dying so fast - 'retreat' and lay siege to castle again - rinse & repeat until defending forces are small enough you can just rush them.

Best achievement was taking a castle from the Zann Dynasty who had 1,700 odd troops with my own force of just a touch over 200. :)

Why would rebel when you only have 1 castle?
Last time I played Perisno myself I owned like 8-9 moderately garrisoned castles (I was a vassal of Geldar and Rhinio just wouldn't stop giving them to me, and he and Oroe helped me a lot with capturing said castles, it was the first time in M&B when I felt like a horrible person for plotting a rebellion against my king) by the time I decided to rebel.
Besides, as a king you can hire mercenary companies and...well... to say that I made some of them a bit OP is an understatement.

Own at least some castles by the time of rebellion and either have lords from dying factions who are your friends and would be willing to join you or make some of your companions lords (some of which are, again, pretty much OP, like Aracanus or Shi Jin).