General view over teritorial strategies

The way you define a territory as yours and you lay out the defenses is definitely a decision factor in the outcome of the game.
This must happen quite quick, by assessing the good and weak points of your adversary's civilization, and of course the type of terrain you fight on.
Defensive strategies are the hardest to achieve, since there is no ""bulk of the armies" and you are more or less scattered and your focus is on economy.
Offensive strategists have the advantage of not worrying about the way they defend their territories as long as their armies are moving fast enough to pose a permanent threat to others.
The difficult part for them will be watching their backs and using the terrain layout efficiently and in a faster pace.
Forests are thought to be a natural wall of defense. Not if you are late in the game they're not. This is why your territory must be in constant surveillance.