If you are playing a Linked Game of Oracle of Ages, then after you complete Level 3 Moonlit Grotto, you will find Impa and a homing pigeon in front of the Black Tower entrance in Lynna City. Talk to Impa and she asks you to rescue Zelda from the tower.
After talking to Impa, go into the Black Tower. Inside, Vire will say that he has kidnapped Zelda, then he'll go up the stairs. Follow him.
What follows is basically just like the game of Donkey Kong. Rolling fireballs will move downward through the area. Sometimes they go down the ladders, so be careful. The fireballs damage you for a whopping two hearts each.
In the upper area, just walk up to Zelda's cage door to open it. Vire will fly away and Zelda will come up to you and thank you, giving you a ring.
Outside, Zelda will say that she came alone to Labrynna because she saw a vision of a world filled with despair. Impa tells you to go northeast if you need to reach the mountains.
Go get the ring appraised that Zelda gave you. It's the Blue Joy Ring, which makes recovery hearts heal you for two hearts instead of one (including hearts found in pots and other sources, not just from enemies like the description suggests).