How come the forts are free to build?

[Ludens]

As i see it, in EB forts are not used for your armies to rest on the march, but to block access for enemy units. I usually occupy a fort with a unit that is almost depleated, and it will hold the AI for 2 turns.

I see what you mean, but this is only a problem for the A.I. You can limit yourself to one fort per access-route, and the problem is solved.