why do the Iroquois have fortresses anyway? I mean a fortress doesn't entail a wood fence or something, you're talking about a serious fortification. If most of Persia isn't fortified I don't see why every native tribe has totally fortified territory. *Maybe* the Aztecs could have a couple on their important provinces, and the Maya maybe, but the rest?
Personally I think it was a mistake to make the north american tribes playable. They didn't have any cities, they were organized yes, but still tribal. EU though is a city based game, each province has the City, that you fortify, hire tax collectors, build armies and garrison with defenders. While central american civilizations had cities like this, the Iroquois for instance didn't collect taxes, didn't live in cities, didn't have armies (during wartime the Warriors would be summoned to war, but they fended for themselves, and were not paid per se) and certainly didn't fortify their settlements to the degree that would allow for a fortress.
What should have been done is create a new form of government, Tribal, where you don't have a capitol, or a city, just a province with a population, trade and production cash, and no taxes. You have most of the diplomatic options available (war, peace, alliance, etc) except diplomatic options with more centralized aspects don't count, like trade embargo, royal marraiges, vassalization, etc. A more advanced civilization can place trade posts inside a tribal nation, though during wartime you're likely to lose them. Tribal societies cannot build fortresses.
Tribal warriors are created naturally over time in provinces depending on your population, and the number of warriors you have as a standing army. In wartime the ceiling is raised, and at peace, the warriors desert to the level of the previous maximum. In war, if a civilized society holds a tribal province for a full year, that province becomes empty, and the previous population becomes natives in the province, with a level 1 colony. If a tribal army captures a province from a neighboring tribal nation, and holds it for a year, it becomes theirs, while they must negotiate for peace with civilized societies. Tribal societies can become 'civilized' when they achieve infrastructure level 0 (takes a lot) then they can build cities in their provinces (100 ducats). The first city you build becomes your capital, and you are now a Civilized society, this is good because you can now fortify your cities, it is also bad because you no longer get free warriors. The city you build is 1/10th of the provincial population. A centre of trade without a city generates no special benifits to the nation who owns it.