Hey, I was confused too, but here's what helped me.
I had to go to the blueprints table and select the "Fortifications" tab at the top of the menu. With that option selected, you will see that the fortifications can be built. I then had to go outside of Fort Forinthry and circle around to the east side of the walls. From there, you will have building nodes.
Hope this helps!
-aqua
24-Jun-2023 14:54:44