Learn Turkish: En üst kattaki teras neresi? - Where is the terrace on the top floor?

Nerede means "at where" and is used to ask for the location where something is happening or physically placed. Neresi, however, is a noun meaning "what place" or "which place". Here, you are asking to identify the place itself ("Which place is the top floor terrace?"), so neresi is used as the question word.

How does the word kattaki break down?

It is made of three parts: kat (floor) + -ta (the locative suffix "on/at") + -ki (the relative suffix "the one that is"). Together, kattaki acts as an adjective meaning "the one that is on the floor". This connects the location directly to the noun teras.

I know üst already means "top" or "upper". Why do we need en before it?

En is the superlative marker meaning "the most". While üst kat just means "an upper floor", en üst kat literally means "the most upper floor". This is the standard Turkish way to specifically say "the top floor" or "the highest floor".

Could I change the word order and ask Neresi en üst kattaki teras?

Yes, but the current word order is much more natural. In Turkish, the most emphasized information goes right before the verb, or at the end of a "to be" sentence that has no hidden verb. Putting neresi at the end properly emphasizes the question word itself.