Final Fantasy IV/Return to Baron

Fight Yang
Exit the building of the Serpent Road and you will be back in Baron. Go to the Inn in Baron and climb to the top floor. There you will find Yang. Speak to him, and he will engage you in battle. Simply attack a few times, survive the battle and Yang will surrender, and join your party having been under mind control. He will also give you a key to a passageway to lead you into Baron Castle.

Enter Old Waterway
On the far left side of the town of Baron, you will find a locked door. Use the Key of Baron on that door to enter the Old Waterway. The key will also open the Weapon/Armor shop if you would like to buy armor before travelling.

Enter Old Waterway
The path through the first level of the underground is pretty straight forward. Make your way around the waterway, collecting any chests desired.

B3F
In this level there are secret holes in many of the walls that will let you pass through. First, head through the wall on the right, and then head through the wall on the right again. When you reach the large area, there are 4 rocks arranged. Go in between them, and pass through that wall. Once through the wall, go north to the door.

Exit
In this room, simply go to the right, up the stairs and through the door. In the next room there will be a door straight ahead, where you can save your game and use a tent. Once you are ready, leave the room with the save point, head to the right, and up the stairs. Once outside you will be at Baron Castle.

Baron Castle
Go around the castle and up the stairs to get inside the castle. Go through the door on the right side right inside the castle. Now go up the stairs and down the next stairs until you reach the main room. When you are walking forward you will be approached by Baigan. He will appear to join your party, but in actuality he will engage in a fight.

Baigan
In this battle, after you attack Baigan with the first magic spell, he will use a wall so that you cannot use magic spells on him. However, the Twin magic of Palom and Porom will not cause him to cast Wall, so this is a very effective technique. If you want to use other spells, have Tellah use the Wall spell on someone in your team, then use an attack spell on that member. Once you destroy his body, you can destroy his arms. Also, if you kill his arms, he will simply regenerate them. So you have to attack his body first.

Kainazzo
Once you have defeated Baigan, continue straight ahead into the throne room. When you confront the King, he reveals that he is Kainazzo, Fiend of Water. He is weak against Lit, so have Palom and Tellah cast Lit on him. When he starts to "Gather the Water" you must use lit, so that the wave will be drawn away, otherwise he will release a very damaging "BigWave" on your party. When he retreats to his shell, your attacks will be less effective, and he will be able to cure himself.

Cid
When you defeat Kainazzo, Cid will storm into the room. He will lead you out, but you fall into a trap. When the walls close in, Palom and Porom turn to stone to save you. At this point you view a cut scene of Kain, Golbez and Rosa. Once that ends, he will take you to the airship. Kain will fly toward you and offer you Rosa in exchange for the Earth Crystal of Toroia. Now you can head over there.