Then if they are higher than level 2 they can be added at game start or by event.
BTW if you want to get all historical tell me where super forts in WWII actually made much of a difference. Most of them were maneuvered around, bombed into rubble or penetrated at a weak point. The only super fort I can think of that actually put up a decent fight was the Russians at Brest. The Germans opted to siege it send the bulk of thier forces around it. And if memory serves me correct German special forces took out both the Neatherlands and Belgium super forts within 48 hours.
The entire purpose of forts is to enable economy of force actions. Meaning you can dump third rate under equipped divisions without prime movers on those sectors and they will hold while your better equipped troops fought elsewhere. Forts did exactly that throughout the war.
France spent 5b francs on the Maginot line, peanuts compared to what they spent on defense elsewhere and half what Germany spent on the Westwall. It was not penetrated until after the Battle of France was lost, and then only in a weak point in the Saar where the field elements had been pulled off to deal with the existential crisis going on to the rear.
The extension was penetrated at Sedan, but it was not completed, and was staffed with poorly trained soldiers led by officers who frankly did a poor job of handling what should have been a straightforward action. The French army had expected a German attack on the Maginot extension (albeit not quite so strong of one) and supplied their garrison formation with a strong mobile corps to beat any penetration back. It was not used as it should have been, to crush the bridgehead. This is a book length topic though.
Forts are not and were not seen as war winners. Their purpose was to enable concentration elsewhere and they were very good at it. Thanks to the Westwall Germany was able to safely concentrate all of their best forces on the attack while leaving a skeleton force to defend their weak flank.
As far as your question regarding where forts were successful, people didn't generally attack fortified lines unless they thought they had a serious advantage.