Wonder Boy in Monster World/Purapril, the Aqua Capital

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. It takes 8 hits with the Gradius to finish him. After you've defeated him Princess Sheila Purapril will be released. She thanks you and ask you to come to her kingdom.

Climb this tower's long staircase to reach her Kingdom. Once there, head right and talk to the first woman you see. This will unlock the door to the next town. Keep heading all the way right until you reach the Princess, and talk to her as well. Now go all the way back down the long staircase and enter the first door you reach (the one slightly to the left), which is not the door you used to first enter this tall tower. Proceed left and 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.