In a medium bowl, mix together crushed pretzels, 1/2 cup sugar and melted butter until blended. Press into the bottom of a 9x9 inch baking dish.
In another bowl, beat 1/2 cup sugar and cream cheese until fluffy. Fold in whipped topping. Spread over the pretzel layer in dish.
In a small saucepan, stir together cornstarch and 1/4 cup sugar. Mix in reserved pineapple juice, and bring to a boil over medium heat. Cook, stirring until thickened, about 5 minutes. Stir in crushed pineapple. Spread over the whipped layer in the dish. Cover and refrigerate for at least 1 1/2 hours, or until firm.