Most likely, the road network is created after the placement of objects, of which there is a fixed number for each zone (castles, mines).

Yes, the road network is definitely created after the placement of objects. In the presentation by the Heroes developer, Smedstad, it is stated that roads are laid along the shortest paths from city to city and to the shipyard (a pathfinding algorithm is used to find the shortest distance from a city to objects, including teleporters and elevators).

Regarding mines, a wood mine is placed first, then an ore mine – at a distance of 12 cells from the castle, if possible. If not, then as close as possible to the available location.

Objects are placed by type: first cities, then mines, then treasures. Moreover, each type of object is placed as far as possible from the same type of object that was placed earlier.