LINE 9-inch square pan with plastic wrap. Arrange cake slices on bottom and up sides of pan. Add boiling water to gelatin mixes in large bowl; stir 2 min. until completely dissolved. Add ice cubes; stir until gelatin is slightly thickened. Remove any unmelted ice. Whisk in 1 cup COOL WHIP until well blended. Refrigerate 15 min.
POUR gelatin mixture into cake-lined pan; cover with plastic wrap. Refrigerate 4 hours or until firm.
UNMOLD dessert onto plate; remove plastic wrap. Frost dessert with remaining COOL WHIP. Refrigerate until ready to serve. Decorate with ice cream cones and candies to resemble a castle.