Learn Polish: Najwyraźniej w tym zamku straszy. - Evidently, this castle is haunted.

Yes, but it changes the meaning. Najwyraźniej means "evidently" or "apparently"—you are making a deduction based on evidence, like hearing weird noises yourself. If you used podobno, it would mean "supposedly" or "they say"—meaning you only heard a rumor that it is haunted, but have no direct evidence.

Why is "is haunted" translated as straszy? Isn't that an active verb meaning "scares"?

Yes, straszyć literally means "to scare" or "to spook". In Polish, to say a place is haunted, you use an impersonal construction. Literally, the sentence says "it scares in this castle" (w tym zamku straszy). You do not use a passive voice like in English.

I noticed zamku drops the "e" from the nominative form zamek. Is this a common pattern?

Yes, zamek has a "mobile e" (a fleeting vowel) that disappears when you add case endings. Because the castle is the location here, it takes the locative case after the preposition w, making it w tym zamku. You will see this dropping "e" in many masculine nouns, such as pies becoming psa.

Can I place najwyraźniej somewhere else in the sentence?

Yes, adverbs like najwyraźniej are quite flexible. You could say W tym zamku najwyraźniej straszy, which is very natural and shifts the topic slightly to the castle itself. Putting it at the very end is also possible but sounds more like an afterthought.