Forts in M2TW are useful in the exact same way they were in RTW: to block strategic points with a small garrison. The most effective fort use I have ever seen was one I engineered in a RTW:BI WRE PBM. Check out the following post and you'll see specific screenshots showing how forts allowed me to secure the entire European border from the English Channel to the Black Sea with only 4 armies.

https://forums.totalwar.org/vb/showp...9&postcount=32

In M2TW the choke-points are different, but there are many places that could benefit from a similar system. The Pyrenees, Alps, and Danube are borders that could work like this.