Learn Polish: Ów władca mieszkał wówczas w zamku. - That aforementioned ruler lived back then in a castle.

While ten simply means "that" or "this", ów is a formal, literary word meaning "that aforementioned". It points back to a specific ruler who was already introduced earlier in the text or conversation. You will mostly encounter it in literature or formal storytelling.

Since władca ends in -a, why does it take the masculine form ów instead of the feminine owa?

Even though władca (ruler) ends in -a and declines like a feminine noun in the singular, its actual gender is masculine personal. Adjectives and pronouns modifying it must always match its true gender, which is why we use the masculine ów. Another common example is mężczyzna (man) taking ten, not ta.

Is it a coincidence that both ów and wówczas appear in this sentence?

Not at all! Wówczas (back then, at that time) is actually built from the preposition w (in), the demonstrative pronoun ów (that), and the noun czas (time) — literally meaning "in that time". It perfectly complements the formal, storytelling tone of ów.

Why does the locative form of zamek end in -u (w zamku) instead of -e?