"Where is the room?"
I think it has in part to do with the certainty/specificity aspect of the question. Ga is sometimes used when it is unclear whether a thing exists or is generally inspecific, whereas the sentence somewhat assumes there is a room and a specific one at that--you just don't know where it is. If you were asking is there a room at all, ga might be more reasonable depending on context. Ga vs. wa is much more complicated, but I think that's the reason in this particular case.
Check out http://nihonshock.com/2010/02/particles-the-difference-between-wa-and-ga/, I found the explanation helpful.
I was wondering the same thing and all I can figure from the context is that a room can't move, so we have to use ですか, while the カメラ can be moved, so it is able to use ですか or (に）ありますか。I'm just guessing based on context clues, so hopefully someone more experienced can explain it better.
while grasping the concept of the topic is difficult; if you do very litteral translations with "as for ..." you can see why it is "room" and not "where" that can be the topic:
どこは部屋ですか。(as for where, is it a room?)
いいえ、質問語です。(no, it is a question word)
部屋はどこですか？ (as for room, is it where?)
階段の隣です。(it is in the vicinity of the stairs)
'Cause you're asking about the room, it is the topic of the sentence, and not "where". It's a question word. And I also learnt that before です, you do not put any particles
I'm a learner as well sorry if I said anything wrong
you cannot mark the unknown with a topic marker「は」。 A topic is something that should be known (at least conceptually) between the speaker and the listener, so question words don't fit that category since they are... well, question words.
「どこが部屋ですか？」should be fine though, it's more like saying "where is the place where a room exists?"
I think because が is normally used when something is unknown to one of the speakers ( 部屋があります, the second speaker did not know that there was a room before being told), and because it is being asked WHERE the room is, I assume that because the speaker knows OF the room in general, は is used.
I like to make up a visual story for each kanji, like the second character 屋 looks like a table in a room with a tatami floor, with a roof over it, etc.
部 looks like a building with a flag beside it, etc. Like a department for something or a designated place.
Works well with stuff like 島 (looks like a bird sitting on a mountain, and birds like to rest on islands)