Heroes of Might & Magic III: Horn of the Abyss

Okay, but would it be possible to implement a feature where a player can build a castle in any location on the map, provided they have the necessary resources? I like the idea that, for example, there is a narrow passage between mountains leading to a large number of mines. It would be impossible to guard all the mines, so you could build a castle in the passage, and the enemy would not be able to pass without fighting the castle. But then there is another small problem – it would be necessary to make it so that players can exit the castle from any free side – this is also a very important innovation, especially tactically when leaving the castle.

but then you should make it so that building a city costs incredible effort for the player, otherwise – it will be unbalanced again!