I always use them to block choke-points. As the Julii for example, you can seal Italian Gaul off from attack through the alps with a couple of well-placed forts. You only need to man them with a unit or two of militia and the AI leaves them alone every time. I have yet to have a fort attacked by the AI.
(I also managed to completely seal off Asia Minor from attack by land from the east with half-a-dozen forts in the valleys - playing as Pontus.)
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