[Gateway103]

One possibilty I suppose is to have a "preference" and/or "discount" for a choosing fortification type. This could be faster build time, cheaper cost, morale boost for garrison units, so on. And penalties otherwise.

Alternatively, you can restrict possible type placement based on terrain. E.g. all are buildable at grassland, but Fortified City/Mausoleum not allowed in Forest, so on... While a nationally choosen type ignore such restrictions (or is buildable but are higher prices than normal).

My apology if these ideas were presented already.

-Gateway103