I have been trying to take over all of the forts undetected in Far Cry 4 but I had trouble in the northern ones (Pagan's and Yuma's) but somewhat easier in the southern forts (De Pleur and Noore). This was largely due to the finding of somewhat secret or at least less direct entrances.
In Noore's fort you can climb up the mountain and grapple into the cave below. In De Pleur's fort you can find a hole near a broken dock that will lead you to a ladder that comes right into the fort.
I looked (though not very hard) in the other two forts and found nothing.
Are there secret entrances like these in the northern forts?
Best Answer
From the Wiki:
http://www.ign.com/wikis/far-cry-4/Fortresses
Pagan's Fort: "Feel like avoiding the front courtyard entirely? From the main gate, turn left and take the dirt path to the northwest, which leads to a cave that brings you up, through the gulag’s cells, and lets out just a few feet from the alarms."
Yuma's Fort No major shortcuts listed in this one.