Minor annoyance with Cave Forts

[tibbs]

I built a fort in a mountain in a MP game as MA Argatha. The province had both the mountain and forest icons. Unfortunately I can only build a forest castle there. As Argatha, I am trying to build more cave forts than other types to make use of the darkness effect.

Does anyone know how the fort type is decided in provinces with multiple terrain types? It would be nice if Argatha would build a cave fort in every mountain province regardless of the other terrain types in that province.

Thanks,
Chris