The trouble with forts is that they are always besieged when attacked. Which means the garrison is immediately trapped. And it can be costly to sortie out of the gate under enemy missile fire.
Whereas an army in the open has the option to retreat, and join up with reinforcements, or if it fights, can deploy as normal before the battle.
So with forts, you either have to have very strong garrisons that can fight off almost any attack, or the garrison has to be regarded as expendable and must stand and die to buy time for another army to be formed.
With this in mind, sometimes I find forts useful, sometimes not. Usually not useful at all in the early game when money is tight, since I don't regard any unit as expendable at that stage.
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