The King's Castle is a big place with lots of things of interest. Below is a map of the castle with many of the points circled and labelled, with explanations below.
Enter the castle at the main entrance and walk forward. There are two rooms; one with a chef and one with a scientist. Talk to the scientist. The scientist's name is Gunnar and he will join your party. He is an inventor.
To the left and right of the main entrance are staircases going up to the second floor of the castle. Go up the stairs and walk forward to meet the King. The King is skeptical about monsters roaming the land, and his skepticism is reinforced when his son and army commander, Faelon, calls the theory ridiculous. The King is no help right now. Gather equipment at the shop. At this point you may need to head back to the west and level up in the forest before taking on the mountains. You will need several Holy Water's, Elixir's, and Curing potions for that task.
It's now time to leave the castle and head to the mountains. If the mountains prove to be too difficult at your current skill level, do some levelling up (grinding) in the forest across the sea -- the captain is tireless.
