"The white castle"
Accusative case in Turkish is only for definite direct objects. That means a verb in the sentence is acting on the noun- the castle is being seen, found, known, cleaned, etc. But in this translation, the castle is just there... nothing is happening to it, so we use nominative case (no suffix or changes at all).
The Turkish course seems a little short on Contributors and MODs at the moment. They are volunteers who are also responsible for going through the "Reported errors" and the reverse course (English for Turkish speakers). All of this in their "free time"! (Shout out of appreciation to AlexinNotTurkey for keeping it going for 3 years and counting!!)
Because you're level 20, feel free to go back to some lower skills in the tree to answer any questions you feel confident about. Duo is a free learning program, so it's largely up to learners to help each other. :-)
Totally get that this is volunteer, and I have learned a ton through the forum discussions. I have no real gripe. Thank you for this site. On another note, I am also brushing up on my Spanish and have noticed how much more polite the Turkish learners are as compared the the Spanish. Just my observation.
I've also noticed the lack of netiquette in some other course forums. My assumption on the Spanish course is that there a lot of younger learners supplementing their school courses, while Turkish tends to have a mostly adult demographic. At least I HOPE the rude comments over there are from kids who don't know better, haha!
Sick that even in Turkish and apparantly in Arabic the word for white starts with b and has an a and/or e in it. Just like in Slavic languages, Romance languages and even Chinese where it is 白色(báisè).
The Japanese is btw howaito,so it seems that the two forms blanc/white are very widespread for the word for white.