If you have read the design suggested idea that has the following features.
Factions or clans create an area that has a perimeter guarded by hired guards that build defenses. And players can build 'forts' in outer areas that can be sieged / attacked. And the details that go with that idea, supply, village NPCs, and Factions being city states with 'Forts' in unclaimed territories.
You have to ask the question of sieges and supplying forts. And tunneling becomes a possibility, so building a fort includes a 'floor' that is as strong as walls, since sappers would try to attack by tunneling, and that would include, traps and counter attack tunnels(sallying), and supply entrances could be built as tunnels into fort. And supply cisterns and supply areas could be under fort if secured by walls also.
At the same time, some would try to resupply a fort with underground tunnels, so siegers would also have to be digging looking for supply tunnels (with some detection abilities as skills to make it interesting) (see: ww1 tunneling warfare)
It would add much to a fort defense if you have to also protect from underground attempts to dig under walls or even dig to attack other tunnels. (Although I think the idea that caverns being built without requiring arches or columns or domes for support takes something from the engineering, I understand the reason for that. however that could be added in at some future point.)
Note forts would be in areas of less 'resources underground' so that players could be nudged to build factions in some locations, and have areas between factions that form disputed territories that would have forts fighting for the area.
And controlling that disputed area, could gain/protect some management of an area adding some farming like resources, if they had ability to protect those farmers if an area is sieges (bonuses from an area around a fort, would require fort to have supply's stored up in fort for a siege, and ability to house peasants during siege. And could be challenged by anyone else that puts a fort in area, or sieges a nearby fort.
Zathurus
Anyways, somewhere to post ideas, and if 'ownership of land' is included in game, disputes over ownership would be part of the fun, and that would be sieges, while a player could also have a safe area to build and supply disputed areas. (see zones of control, and Front lines, in Medieval wars.)