Granny Smith apples are used because their high acidity strengthens their pectin, helping the apple slices hold their shape as they cook.
Let the apples stand in the batter for about 15 minutes before baking; during this time, the apples will release liquid and loosen the batter to the correct consistency.
The apples naturally flatten out under their own weight and align themselves against each other, creating the beautiful layered effect all on their own.