Slitherine

How do you do it? I've done just about everything in the game, but I can't figure out how to build an entrenchment (that is, the hexagonal fort that provides additional defensive capabilities- such as Sevastopol or Tobruk). I know it's possible because the AI does it during scenario play near Kursk and on the Atlantic Wall.

Forts are only pre built for the scenarios, they cannot be built manually once scenario start