[MaxWilson]

To elaborate: in the manual, it says which fort types your nation builds in which kinds of terrain. (When there are multiple terrains, forest is preferred over mountain. I'm not sure about the order of the others.) Fortified City is usually a plains/farmland city for most nations that have it, while Hill Fortress is usually in the mountains. Hill Fortress gives better defense and is cheaper but boosts income less (doesn't matter much) and collects less resources from surrounding provinces (but that's usually offset by the hill fortress province itself having more resources). I like Hill Fortresses better in most ways.

-Max