I use paint for most of my pixelart, a lot of great pixelart is. Definitely not the best software to use for it, but
its still better than a lot of other programs.
As for hyrule castle, it doesn't look too bad, but I don't really know whether or not I would classify it as pixel
art. There are some great tutorials and guides online that explain the differences, I would suggest you look
at some of them.
Here's a really good one I found.