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.

Rescue Zelda

After you talk 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 much 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 two hearts each. Equip your Roc's Feather and you can jump over the fireballs as they pass by.

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, and she will give you a ring.

Outside, there will be a cutscene.

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).

Afterward, you can go west from Lynna City to proceed to Symmetry City.