I can't recall ever hearing a native speaker pronounce the T in 'castle'; it's typically silent. You're correct that some native speakers pronounce the T in 'often' while others don't. There is some disagreement about which versions is correct: http://www.dailywritingtips.com/how-do-you-pronounce-often/

Yes. If you ever wanted to know how some word is pronounced use google use the following format define: castle define: often The first search result will show you word and there will be a speaker symbol. If you click on that speaker google will pronounce the word for you. Hope that helps Cheers. AK

In my experience most people from the UK, and especially from England pronounce the 't' in 'often. Myself included. I have never heard a native speaker pronounce the 't' in castle.

The t in 'castle' is always silent. However, some people pronounce in the t in 'often', I am one of those people. But you won't be wrong either way in this case.