No, it will be exponential because each new city will add to the already existing cities ...

1 city with mine = + 5 iron

2 cities with a mine in each = (5 + 1 from the town B ) + (5+1 from the town A) = 12 (and not 10)

3 cities with a mine in each = (5 + 1 + 1) + (5 +1 +1) + (5 +1 +1) = 21 (instead of 15)

4 cities with a mine in each = (5 +1 +1 +1) + (5 +1 +1 +1) + (5 +1 +1 +1) + (5 +1 +1 +1) = 32 (instead of 20)

5 --> 12 --> 21 --> 32 etc... is an exponential serial.

5 --> 10 --> 15 --> 20 is an arithmetical serial.

And I didn't take into account the caravans .. or the fact you can have pultiple mine in a city.

You can also use reverse thinking :

the first mine adds +5 iron

the second adds +5 iron + 1 from the other mine.

the third adds +5 iron +2 from the other two mines.

the more mines you get, the more they produce. Even if I like that idea, it will become a nightmare in the end-game.

One more thing : the caravans. A caravan is created for every link that can exists in your kingdom ! that's crazy.

With 2 towns : 1 caravans

3 towns : 3 caravans (a-->b, b-->c, a-->c even if a, b and c are like this : a------b------c)

4 towns : 6 caravans

5 towns : 10 caravans

6 towns : 15 caravans

7 towns : 21 caravans

8 towns : 28 caravans

... <<<some calculations later>>>

20 towns : 190 caravans ... Isn't it a bit too much ?

I think caravans should be limited to towns directly connected. With the case of A------B------C there would only be 2 caravans (A-->b and B-->C)