Yes and no.
To start a kingdom, the PC must throw over his loyalty oath, which is a capital crime. Such it is an endeavour that should not be taken lightly. You'll need a PC who somehow grow powerful enough to sustain the backlash.
That is the Julius Caesar's thing: "If you must break the law, do it to seize power: in all other cases observe it."
Part of it is to get the support of some other lords (warband)
Does it require planning? Not really. There is no perfect path to a kingdom, only paths that come with consequences.
The PC starts as an unknown character who wishes to make a name for himself/herself. Once she/he is reputable enough, Kings pay attention and wishes the character to turn into a vassal.
In the doing of acquiring that reputation, the character is given chances to start befriending lords. 30 relationship to a lord is the critical point. Up to 30, lords considers the character as a mere errand boy/girl. From 30, the relationship is changed, they no longer issue quests and start to consider the character as a peer in value.
So 30 relationship to a lord is the critical mass. A lord with a relationship over 30 is material to join a coup. Under that point, the lord is difficult to manage.
Reaching 30 relationship with many lords comes naturally if you dont rush the getting known stage. Taking one tour as a mercenary for a faction and joining another faction as a vassal is usually enough to get enough of lords over 30 reputation.
Another thing is that reputation carries over deeds. Other lords, even when not in the faction or the action, might hear of a deed and approve it therefore the relationship to that lord is increased (the game was supposed to feature mechanics to make enemies but it never really made it into the game)
Worth keeping an eye on the report log as those lords'name is reported anytime they approve of an action.
In this game, behaviour is taken into account and the game's experience follows.
If you rush your kingdom building, you'll get the experience of an impetuous lord who can not wait and has to be satisfied at once.It is neither good or bad, the experience that follows fits the behaviour.
If you carefully plan your kingdom building, it is also neither good or bad. It gives the experience of a planner who is will get frustrated at times as things do not come together as they were planned.
Same things for the choice of vassals: if you go for a homogenous band, they wil act as one man. If you take pitiless lords, they create more casus belli and expect the king to follow. If not, you appear weak and alienate them all.
A heterogenous band affords more flexibility, they are less cohesive though.
No planning required, simply the consequences associated to certain behaviours.
Another example, if before turning into a vassal, you choose to build up reputation through battles, neglecting to build up your relationship with lords,village and cities, and that, once a vassal, you seize the first opportunity to break your oath, you'll get the experience that follows the behaviour.
You'll be strong in battles, but will have to rely on yourself mostly.
In this game, there are no perfect paths, only paths and any time you'll face the consequences (good and bad) of your behaviour.
