Yes, that is the magical piece.
However, you should be able to use that piece like this:

or this:

or:

I could be wrong, but I think it is the same piece as here:

Now, I'm going to have to look - no, I remember, it is the same. Yes, you can't use the brown piece, but you can use the castle top. Mind you, that's what you're asking about to being with not the top ...
You could email DS and ask if they can get the posts. Or pay outrageous ebay prices ... (unless you get them as part of a lot, which would most likely include another brown piece anyway ...)