After falling in the gap (mind the gold in the grass left from the ladder) enter the door, walk to the next door and enter it again.
You are now in the underground passage of Purapril castle. Walk to the left, after a while you'll encounter a knight. He can be easily defeated by striking at his feet. Use the spear if you have it, as it deals slightly more damage. After you've defeated him Princess Sheila Purapril will be released. She thanks you and ask you to come to her kingdom.
Climb the tower to reach it. Once there, make sure to talk to everyone, they have valuable information for you. Especially you need to talk to the maid near the belltower, talking to her unlocks the guarded door you passed at the beginning of your journey. When you're done, leave the kingdom. Go to the bottom of the tower and enter the door, which was closed before. You're now in Purapril the Aqua capital. This is an important town in the game, you have to return here often. Buy some goods and leave the village through the south-west exit.