Before completing the Underground Castle, you must use the Dream Rod to visit Lisa's dream.
Talk to the Rescued People
Talk to the tool shop owner's son to get a free bag of goat food. The son will try to charge you one billion dollars plus tax, but no matter how you answer, the mother will get angry and give you the food for free.
By talking to the people and animals in town, you learn that the Village Chief is trapped in the painting in the house on the hill. You can enter the painting if you touch it with an object that belonged to the painter.
Before going to Lisa's house, go north from her house to talk to Gaudi, the architect. Once you talk to him, he will begin work on rebuilding the town wall. The next time you leave the area and come back, the wall will be done.
You can find a Medical Herb hidden in the southeast corner of the goat pen.
Visit Lisa's Dream
Finally, go to Lisa's house. She is asleep, so equip the Dream Rod and check on her. You will be pulled into her dream. Talk to her in the dream and she will call out to her father. Step on the cleared monster lair nearby. This will open a blocked path in what appears to be the Underground Castle. After this, a sad voice will tell Lisa to tell everything that she knows to the person in front of her and give him Leo's paintbrush. Lisa will then look around and ask if that voice was her father, but there will be no answer.
Talk to Lisa and she will wake up. She'll kick you out of the house, but after you leave she will realize that you are the person that her father's voice talked about in the dream, and will call you back. She will say that she hid her father's paintbrush somewhere, but if you find it and bring it back to her, she will tell you all that she knows.
Since her dream seemed to take place in the Underground Castle and you just opened a pathway there, you should return to the east part of the Underground Castle.