Rise of the Frosted Lanterns

There are four kingdoms. Two to the Sun. And two to the Moon. But one day, and a year apart, the two youngest heirs are taken for their powers. The years go by, and they grow up in a tower with an enchanted barrier, that allows anyone to enter, but only those who've taken a special potion to leave. Can the two older heirs save them? Or willthey all live in that tower forever?