If you are trying to fortify your entire border then your clearly approaching defence incorrectly.
You want to be able to have large numbers of units scattered across your borders or preferably in between the enemies' borders and your own so the moment any invasion seems likely, you can either attack first or intercept their units.
Having a static line of units is costly and vulnerable to intense bombardment. It would only ever be useful if you had a narrow strip of land that was clearly going to be the invasion path for your enemies.
You might also want to build forts parralel to any cities you have on your borders. If you have a city with perhaps 2 normal units defending and then several bombers or other artillery units within that is on a vulnerable border, go two squares away from it (
so your forts do not get in the way of resource harvesting) and build a line of 3 or 5 forts.
Make the central one so it is in line with the city. This means any unit coming from outside the garrison's line of sight will not reach the city within one move. You will be able to start bombarding them before they threaten the city and if the garrison is outnumbered you can decide whether or not to retreat to your city.
You shouldn't think of a fort as just 'part of a long line of forts', it works perfectly well on it's own. The Great Wall of China didn't work, the Siegfried Line line didn't, the Maginot Line didn't... Long walls do not work, as Speciou5 said (
who is clearly your military superior
), the Hedgehog defence works brilliantly, any square you are not harvesting and is within your territory should have a fort on!
If the enemy only ever has to conquer your cities then you will fall very quickly unfortunately.
The Germans aced the hedgehog defence in Russia and North Africa, perhaps you should read some military history before you lay down the law on how to make war
BlytZ