1.)I definitely think forts should be buildable outside of your territory, or even in enemy territory if you can keep a worker alive that long. My thougths are that making it take twice as many turns to build it outside your territory and making it cost 4 times as many turns while additonally requiring the "Engineering" tech to build one inside enemy territory should amply balance things in that area.
2.) Forts should produce culture like cities, but should also grow like cottages. This is in line with what the person who posted the thread has already put down. I think it's radius should not exceed 1 square, but I still think it's culture growth rate should vary... this is it's influence will not be far-flung like that of a city, but its influence will be very strong on the squares right next to it.
2b.) Growing like cottages would function simply. The defensive bonuses will go up with each tier, also the maximum culture bonus will go up with each tier.(potential names for them could be Fort, Garrison, Keep, and Stronghold... or somethign like that) Additionally, higher tiers would offer defenses geared towards higher tech opponents(many granted only after certain techs are discovered) that slightly counteract some later advantages. Forts would only grow if units are in them.
2c.)Culture should be generated based on the number of higher level units in the fort. If units are in the fort, it will generate +1 per turn. For each unit that is level 4 or higher an additional +1 will be generated. The maximum that could generated would be equal to 2 per tier, making it +2, +4, +6, and +8. Additonally, culture points will not exceed a total of 1000(This keeps people from spamming fortresses to make their culture scores insanely high.) Culture would grant a fort a culture defense bonus like a city, but it will not progress in culture levels.
3.) Ownership of forts will be determined by the nationality of the units in the fort. Whenever it is unoccupied, the ownership is neutral, and no nation without otherwise existing culture in the area will posess the area. Ownership will always belong to whichever nation has it in its own culture border. As such If the fort itself sits within the cultural influence of a city, it will always belong to the nation that controls that city, even if enemy units are in it; however, the fort itself will only exude control over its culture radius when it has a unit from the specified civ camped inside.
3b.) Forts that are in enemy territory will not produce culture or grow as a result of your units. And units of other nationalities do not cause forts to grow or give them culture, although all units inside a fort benefit from it's defensive bonuses. Forts only change hands after being occupied after being vacant.
4.) Religions should spread to forts, like they do cities. They do not count as a city when determining the number of cities a religion has, but they do increase the culture by +1 per religion, but not to exceed the maximum culture output of it's given level. They can spread through traderouts and missionaries, and they are unaffected by the "Theocracy" civic unless the fort itself is inside the culture radius of one of its owners cities. A religion cannot be founded in a fort.
5.) Forts should be able to become cities, in any one of three ways. If this does happen, any religions in the fort are in the city that is created. The city is assumed to have walls if the fort is a Keep or better, or is a Garrison with 250 or more Culture points. If it is a Keep or better and has 500 or more culture points, it is assumed to have a Castle. If the fort is a Stronhold or has 1000 culture points said city starts with a random assortment of buildings, ranging from temples or monastaries of pre-existing religions, to monuments if a level 5+ unit ever based there, to a grocer, or a forge, or perhaps even a library. The city will only have 4 buildings maximum counting the Walls and the Castle. If the fort is a Stronghold and has the maximum 1000 culture points, it has an even chance of starting with 5 buildings. There are 3 ways a fort could become a city:
*A Settler can build on top of it. Any religions, and culture value the fort has will be transferred to the city.
*A Great Person can join a Keep or Stronghold with a culture value above 100 as a superspecialist. This will turn a Keep into a size 1 city, or a Stronhold into a size 2 city, that has said superspecialist. In addition buildings associated with said superspecialist will be put in the city. These buildings do not count against the number or buildings randomly placed in the new city. This is a possible list of Great Persons and the associated buildings. Note that if a building selected can't be built yet, it will not be placed.
--Great Prophet: Temple or Monastery of one random religion. If no religions are in the fort, the state religion or a random existant religion is placed in said fort if there is no state religion, and the Temple or Monastery of said religion is put in that city.
--Great Scientist: Library or Observatory.
--Great Artist: Theatre or Broadcast Tower.
--Great Merchant: Market, Grocer, or Harbor.
--Great Engineer: A Ganary, plus a Forge, or a Levee
--Great General: The Heroic Epic national wonder if not yet built, additonally Barracks, Walls, and Castle are granted, and do not count against the 4 or 5 random buildings.
--Great Spy: Courthouse, Library, and Jail are granted.
It may seem unbalanced to be able to get seemingly free cities out of great people, but great people are nominally more valuable than settlers... and even if the city does get the superspecialist, it probably wouldn't unbalance the game, since this can only be done on older forts that are likely to have already had settlers put on top of them already, addionally, said great person would not be able to join a more powerful city, start a golden age, or create a special building.
*The third and final way a Fort may become a city, is to naturally evolve into one. Each turn the fort has a 1 in 100,000 chance of becoming of becoming city for every culture point it has produced. That is if it has 10 culture points it would be a 1 in 10,000 chance. If it has 1,000 it would be a 1 in 100 chance. Secondly, if it's a Keep or a Stronghold, the total, food, hammer, and commerce value of all squares within the forts culture radius each add 1/1000 per unit to the the odds. If it naturally evolves into a city it has the same effect as if a settler was put there.