He then accuses and proves her a girl, which not only voids her claim to the throne, but is a crime under the old law. That is, to pose a female as a male heir to the throne is the crime.
The jailor is a jerk, & demands all their valuables in exchange for the very basics: beds, food, etc.
There seems to be some inconsistency in the depiction of Sapphire; here, she is depicted as a very young teenage girl. When she dons her costume, she seem much larger.
Princess Knight episode guide


