After activating the elevator, you can continue going through the Underground Castle.
Use the Elevator
Now that you have reactivated the water mill, you can continue in the Underground Castle. From the entrance, go east to the conveyor belt that you created earlier. Then go southeast to reach the elevator. Step onto the elevator and it will go north.
When the elevator stops, you find a lair that releases Pineapple Plants. Kill them and you will release an old man. Now go east to reach the eastern part of the Underground Castle.
Underground Castle, east
Go north and kill the Pineapple Plants. Open the lair to release a goat. This also restores a goat pen.
Go north. There are two Cherry Head lairs here. There are also two Gepper torches. You may find it easiest to use the Flame Ball magic to kill the torches. Then kill the Cherry Heads. The northern lair releases a tulip, and the southern lair releases Lisa.
Go north and then east to find some Water Slimes. Farther south, there is a stone wall with a Pineapple Plant lair inside. Kill the monsters and open the lair to release an architect.
Go south from the stone wall. There are a few Water Slimes and a Gepper torch. Go east from there and walk east along the conveyor belt to reach a section in the middle with a Cherry Head lair and a Bondguard lair. When you clear the left lair you release some ivy, and when you clear the right lair you release the tool shop owner's son.
Go back to the conveyor belt and walk onto the conveyor belt that goes north. Keep going north until you reach a Zip Fly lair. Clearing the first lair creates a second Zip Fly lair, which creates a third one, which creates a fourth one. After clearing the fourth one, you release a goat.
Finally, go east from this room and then go all the way south. There are some Water Slimes here. Once you kill those, go west. In this narrow corridor, there is a Cherry Head lair. Clear this lair to create a Zip Fly lair. Clear that lair to release a tulip.
Go west from there to find a Cherry Head lair. Clear that lair to open the wall to the left, creating a shortcut to the exit.
Go south and walk east on the conveyor belt. You will find a Gepper torch in the corner. Continue going east and you will find another Gepper torch. Kill it and go north, then west. There are two Gepper torches and a blue crystal. Kill the Bondguards that come out of the lair to release an old man.
Back in the Underground Castle, kill the torches and then talk to the blue Crystal Fairy. She will tell you to come back with a better sword in order to kill all of the monsters. She will ask if you want to go back to Grass Valley. You have cleared all of the lairs that you can reach, so return to Grass Valley.