The Phoenix and the Carpet (1997)

Episode 4 (1x4)

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As part 4 opens the children use a third wish to get Robert out of a jam. They soon discover that the tower is totally sealed up. They find an old door that has been covered by a wall. They manage to break the wall and door open and the boys go exploring outside. Inside the girls find a sealed box. Whey they pry it open they discover it is full of gold coins.