The Iron Fortress will remain open every two weeks during a specific period of time. Check the countdown in the purple event widget (left side of the playing screen): it will appear before the event starts. 
Since each event provides a new Legendary character for your regular prisons, each event should restart to achieve the proper challenges.
You will be able to win useful rewards depending on the prisoners you rehabilitate during the event. Every week you will have the opportunity to unlock very special characters!
Event chests will give you some permanent upgrades and the possibility of unlocking the Master Key. This special card will provide your Fortress with great advantages during the current event.
Managers purchased in your regular prisons won't work in the Iron Fortress (only the no-ads will), but they will work forever in your permanent prisons.
Event purchases made during the event will only be available during the event where they were acquired.