Once you finish every objective in a world a secret location will become available. Keep in mind that each Quest you play has 3 objectives.

To get the most out of your secret location try to open it as far into the campaign as possible and on the hardest difficulty level.

Be sure to match the Talisman with your character. That way you'll be able to make to most out of it.

Use lock Character option so you don't sell your main characters by accident.

Evolve your characters as high as possible early in the game. Maxing out their skills will unlock their full power potential.