Do you want to explore the fortress on your own?
Then pick up the children's audio guide at the museum shop. It's a device with headphones on which short stories are stored, similar to an mp3 player. Whenever you see a sign with headphones somewhere in the fortress, you can find out more about that place by pressing a button.
If you have a smartphone with you, you can also simply call up the audio guide online and listen to it - free of charge and without installing an app.