This has to be unlocked in adventure. I personally prefer wordament, though jumble and crossword are also possible. Keep playing levels and unlocking enough stars to both reach the gate and open it. I needed to complete 20 levels and earn at least 50 stars. Combined with the bonus level I got 63 stars.
Use the wordament solver when you need a little help finding words:https://words-solver.com/wordament-cheat/