Oh Isabelle?
So you'll need to head to the Castle. To get the key, you need to get the blue slime from the tent in the Forest. Beat her or lose to her 3 times. If you eat her you still get the key but I'd advise taking her with you. If you take her with you, wait till it's a day divisible by 3 and she'll ask for your help. Complete the quest and she'll give you the key as a reward. You can now access the castle and beat the Queen, which then opens up the next area to go (once you do a couple of things first).
The guide is almost ready. But I'm just gonna write it for the next version which will come out in a couple of weeks.