2D Legend of Zelda style game where the Princess gets sealed inside a giant crystal and the Hero has to save her, as one does, except the Hero manages to obtain the crystal at the end of the tutorial dungeon.
The bad news: nobody knows how to break the giant crystal without killing the Princess in the process.
The good news: the Princess is fully aware inside the giant crystal and can still use all of her Magic Princess Powers within a limited range.
(The Big Bad knows this, and tried to work around it by keeping her in a large empty room with nothing nearby for her to target. That didn't work out so well!)
The whole rest of the game is basically the Hero lugging this big stupid crystal around in order to bring the Princess within range so she can use her magic to actually solve the problem at hand.
The Hero plays basically like Link from a typical Zelda game, except all of the Hero's upgrades relate to making them a better crystal-carrier: throwing the crystal, teleporting to the crystal, doing an AoE attack that affects a line between them and the crystal, etc.
When you switch to playing as the Princess, you're both invulnerable and physically immobile, and the joystick moves your targeting reticle for your spells instead. Those spells are mostly puzzly environment-manipulation stuff with an elemental theme, plus some basic damaging blasts to discourage monsters who get too close. There are recurring sequences where the Hero is indisposed for some reason and you need to figure out how to use the Princess' spells to move her into position to solve the current puzzle and un-trap the hero by using environmental effects and contraptions to fling the crystal around.
(In the final battle, the Big Bad proves to be completely immune to both the Hero's weapons and the Princess' spells, and winning involves setting up some sort of Rube Goldberg sequence of events which causes the Princess to fall on the Big Bad's head from a great height, killing him instantly.)
There's a recurring prophecy throughout the game that the only way to defeat the Big Bad is to turn his own magic back upon him, and long-time Zelda fans are likely to assume that this means some sort of magic missile tennis, but it actually means a different thing.
