I’m not sure if this is a bug, but after completing the game, when I played it again, the gate to the castle was closed. However, I had already used the key, so I couldn’t open it.

I'll investigate it, thanks for the bug report. Was it the door to the castle (opens with the sigil stone) or the smaller door in the throne room (opens with a key)?

Ah, got it. I forgot to save the door state in my code. I'll make an update, and the doors will be open if you've used the key previously.

Fixed it, the new versions are up (both free & the donation version).