Cadastral surveying in Western Europe basically ceased between the fall of the Western Roman empire and the 16th century. Land descriptions continue to be created (the Domesday Book) for example, but the abundance of legal squabbles in the medieval period over who precisely owned some bend of a brook or plot shows that the available land descriptions lacked the prima facie precision required to settle these disputes without interpretation. These is a good case to be made that most medieval landholders who owned more than one manor probably only had rough (if serviceable) ideas of what they owned. , and even within a manner there could be tremendous disputes about where the boundaries of some renter's parcel began and ended. It's not until much later (the Renaissance and into the 18th century) that you start getting enclosed estates that clearly demarcate ownership.
This comes to mind because everyone is atwitter at the moment about Adventurer, Conquerer, King and domain management systems. I haven't read ACK and this isn't a criticism or review of it, so perhaps the system I'm about to describe already exists. If it does, more power to it.
Medieval feudal politics is all about personalities. Land, loyalty, soldiers, money, titles, ancestry, offices and manors are ways that the respective personalities measure and interact with one another. I find that domain management systems often prioritise these measures at the expense of their foundation, individual people.
As a proposal then, I am tempted to design a domain management system that places them back into prominence. When a PC becomes a landholder (whether a noble or whatever else), the map that is presented to them is not the map of their domain, but a relationship indicating where exactly they fit into the hierarchy of power. The relationship map would list names and offices, and be connected by arrows indicating the nature of the obligation owed by each one to the other. These would include the wealth a person is expected to generate, whether they have sworn any oaths of loyalty, how many soldiers they have to muster when called, outstanding lawsuits against one another, etc. Expanding your resources would involve the recruitment and training of individuals bound to you, with new titles allowing more valuable individuals to be recruited.
At the same time, I would create a second version of the same map that serves as a true relationship map. It would include how the various people feel about one another, and their goals, and so on. I might have loyalty and competence scores recorded here as well, with each one a percentile that measures how willing and capable of fulfilling their obligations each one is. Failures lead to failures to fulfill their obligations, with each one suggesting obvious problems - an underling who tries to cover his obligations but is simply unable to (perhaps due to problems the PCs can investigate and deal with), a highly capable but rebellious subject, a right-hand man who is competent and extremely loyal, or a scheming bumbler who the PCs need to figure out a way to get rid of. As well, if you lose the person, you lose the obligations, which means you need to negotiate someone else to take them up, or soak up the loss yourself.
This kind of set-up would allow you to continue running interesting adventures for PCs who own land without needing to threaten the integrity of the domain with invaders constantly, because it throws the spotlight of domain management onto the personalities in the domain, rather than the arrangement of land and number of silver pieces it generates. It transforms the dynastic struggles and feudal contests into dramatic conflict that can be gamed, instead of leaving the DM each week to figure out whether it'll be quarrelsome farmers requiring Solomonic judgments or orcish invaders that the PCs have to deal with.
I suspect I'll be drawing one of these up for Arkhesh shortly, once the lands of Isla de Naufragio and the slave rebellion on Isla Nueva Renna have been dealt with.