Learn Portuguese (Portugal): Como a feiticeira fizera um feitiço, a rainha adormeceu. - As the sorceress had cast a spell, the queen fell asleep.

fizera is the Simple Pluperfect (Pretérito Mais-Que-Perfeito Simples) of fazer, meaning "had cast" or "had made". To form this tense, you start with the "eles/elas" form of the Simple Past. For fazer, that is fizeram. Drop the final -am to get the root fizer-, and then add the new ending, which is -a for the "ela" form.

Could I use tinha feito instead of fizera here?

Yes, absolutely. tinha feito is the Compound Pluperfect, and it means the exact same thing. The simple form (fizera) is mostly used in literature, formal writing, and storytelling (perfect for fairy tales about a queen and a sorceress), whereas tinha feito is what you will almost always hear in everyday spoken Portuguese.

Is there a more specific Portuguese verb for "casting" a spell, or do we always just use fazer?

You can use verbs like lançar (to throw/cast), so lançar um feitiço is also correct. However, Portuguese often relies on everyday verbs, making fazer um feitiço (literally, to make a spell) a perfectly natural and very common expression.

I usually translate como as "how" or "like". How do I know when it means "as" or "because"?

When como is placed at the very beginning of a sentence to introduce a full clause, it frequently acts as a causal conjunction meaning "as", "since", or "because". In this sentence, it sets up the cause (the spell) for the result in the second half (the queen falling asleep).