Also the ability to build Military Forts or Mining Station's as a separate entity from a City ... able to be built an indefinite distance from cities, but with an appropriate distance cost. I would say the distance cost would be some combination of Immediate Resource Cost + Time it takes to build, so a Fortress 50 tiles away would take 10 times as long to build as a Fortress 5 tiles away from a City.
Forts could upgrade to Castles, and then to Citadels (maybe) ... however they would be almost purely for military purposes, only for housing soldiers (and feeding them) not for training new soldiers, unlike the way its handled in Medieval:total war ... your Cities would produce everything, and your forts could be relatively costly Non-Economic installations whose sole purpose is to house soldiers away from the Cities, in a defensible fortification.
If you don't like the idea of these Non-City Military Citadels, then perhaps a smaller scale would suite you better.
Say WatchTower -> Bunker -> Fort. (say for instance, Fort Eben Amael)
a Fort would be able to house soldiers for "in-house" maintanence, as if they were in your own city, as long as there was a "supply" road/open route between the fort and nearest city. This could be an oceanic trade lane if the Fort is along a large river or on the coast, or it could be a land-based road.
Then, if you built a settlement(city) near the Fort, and connected the two, you could eventually upgrade the Fort into a Castle. This would include adding quarters for Nobility and the Court, as opposed to simply Common Soldiers + Noble soldiers ... aka you would be adding room for Retinue, as well as increasing the (relative) defenses.
Once you have a Castle attached to a City, you can upgrade the Whole-Shebang into a Citadel. This would most likely include constructing building (outer Wall) and building buildings between the inner wall and the outer wall, and then you Could do a final upgrade of (final wall) which surrounds the outer wall. Then you could build further buildings in-between the outer wall (middle wall) and the final wall (outside wall). In this way your Citadel would have 3 walls, with a castle within the Inner wall. Also, once you decide to build the city as a "Citadel" your inner wall can't change, it will remain static, and excess "space" for building improvements must be covered by the secondary wall, and no longer the inner wall. And once you build the Final (third) wall, the Secondary wall becomes static. The Third wall, however, will never be static, so the decision to create a citadel is basically protecting those buildings in your Primary wall (and next to your castle) more closely than those buildings outside your primary wall. A fully developed Citadel would theoretically have 3x the protection of its City-proper.
I would suggest allowing Turrets for walls of a "Citadel" ... perhaps only starting as a Bowman in a tower, then upgrading to a Ballistae at Contruction, and then upgrading to a Bombard(Cannon) at blasting-powder.
It would be nice to have special anti-air turrets to be built, although maybe these would have to be garrisoned by Air-mana casters, or something. Certainly a late-game protection against Invasion from the Sky ... where you would either have to make a break in the walls or have to suffer fire by the Anti-Air batteries. Certainly only a late game feature though. It would also be nice to have the ability to add Oil burners to the Gates of your Castles and Walls, if someone breaks open a door, and u have these oil burners, people passing thorugh the doorway will have boiling oil poured upon them.
In addition, a Mining Outpost is merely a resource improvement with a small military guard, an indefinite number of tiles away from a city (like a Fort). It will remain operational as long as there is a road/open route between City and Outpost. Again, it will remain operational as long as there is a military unit present and an open route. The route can be an Oceanic Trade-lane, a large River, or a land-based Road.
A mining Outpost should be able to be built upon any non-food resource (food resources may still require settlements, Optional). I am particularly thinking only Ore-types and Shards. A mining Outpost can build a surrounding walls (optional, costs wood) and can house a small military garrison (50-100 soldiers). Im thinking ... that the soldiers would BE the laborers in such an outpost, so that the mining,ect is done by the soldiers stationed there. Either this would simply cost more (soldier's wages) OR it would have a lesser rate of effieciency as a Citizen-mining improvement. A fort can be built adjacent to the Mining Outpost. Perhaps this synergy could increase both protection of the Outpost, and increase mining effieciency. Perhaps an Outpost + Fort is more efficient than a citizen mining camp. If a city is built near/adjacent to an Outpost, the outpost will become part of the city and run by citizens, no longer "requiring" a garrison. If a City is built adjacent to a Mining Outpost which is attached to a Fort, then both the fort and the mine will become part of the city, and the option to upgrade the Fort to a Castle will become available at Settlement/City level 3.
Unlike in Medieval:Total War, any City can have a Castle (probably can have more than one if they *really* want to), and equally any large city can decide to become a Citadel at any time (although if the city can't grow any larger, than perhaps not, as you would need some buildings in between the primary, secondary, and tertiary walls.) However, large castles and Citadels I think belong in a Settlement of at least size 3. Other than that, I think you can have your Palace/Castle in your capital of whatever size settlement, as well as a Motte/Bailey or Fortress in the rest.
One possibility of upgrading a maxed out City into a Citadel would be an option to simply build your secondary and third walls wherever you wish, within certain limitations. This would be a more expensive endeavor and would provide little-to-no benefit other than safety. However, I think it would be best to leave maxed-out-cities to not be able to go the Citadel route. Cause going the Citadel route beyond a maxed out city seems to leave an over-complication that cannot be rectified. In which case, a reasonable option would be that all OTHER than level 5 cities can start becoming a Citadel. Of course, while this on face-value appears to have similarities with Medieval, youll still be able to build the city as large as possible, you simply need a base to start within your primary wall. If you want a third wall, you will have to Solidify your primary wall at Size 3 and your secondary wall at Size 4. Only in this way will you *most likely* have enough buildable space within your third wall.
Of course, if Citadels won't be in the game, then I would at least like to see some implementation of military-based Mining Operations and Fortresses/Watchtowers without the need for the presence of a Settlement.