Once you've unlocked The Fallen Fortress in 's Everafter region, you will automatically unlock a quest with Flynn that will give you access to the Fairy Tale Trial of the area.
Depending on whether you unlocked The Fallen Fortress or Teapot Falls first, this may be the second or final Fairy Tale Trial for you to complete.
The other Fairy Tale Trials all work similarly to the one you completed to unlock Flynn in . Before you can open the portal to enter it, you'll need to complete the quest with Flynn, which gives you the required Fairy Tale armor.
Once you have that, pull the lever to lower the bridge leading to the trial and enter the portal. Upon entering the Fairy Tale Trial, you'll be faced with a closed door and Flynn on the other side.
To open the door,interact with the knight on the right side to spin it into the correct position, matching the armor on the left side of the door.