Each of the castles had own army of knights.
which pronoun agrees with the antecedent in the sentence?
her
its
their
his

Each of the castles had ITS own army of knights. The pronouns each and every always take either a singular pronoun or a singular verb. So, you can dismiss the option "their" immediately because it is a plural pronoun. So let's take a look at the other two, her and his. Castles do not really have a gender in English, and his is used for masculine gender, whereas her for feminine, so neither of those two options is correct. Thus, you have to use ITS.