Learn Russian: Наверху есть ещё комната.

Наверху answers the question "where?" (location), which fits here because the room is already located upstairs. You would use наверх to answer "to where?" (direction of motion), like if you were walking upstairs: Я иду наверх (I am going upstairs).

Why is ещё translated as "one more" here instead of "still"?

While ещё often means "still" or "yet" with verbs, when it is placed before a noun it means "additional" or "one more." Note that if you used другая комната instead, it would mean "a different room" rather than simply adding a second room to the total count.

Is the word есть strictly necessary here?

You can drop есть, but keeping it emphasizes the actual existence or availability of the room. Saying Наверху есть ещё комната sounds like you are pointing out an available option ("Hey, there actually is another room upstairs"), whereas dropping it makes it a flatter, purely descriptive statement.

Why does the sentence start with Наверху instead of a direct translation for "There is"?

Russian doesn't have a direct equivalent to the English dummy subject "There" (as in "There is"). Instead, Russian naturally puts the location or setting at the start of the sentence, and the new or important information at the very end. Starting with Наверху establishes the scene before revealing what is there.