That's a matter of taste and preferences. As your stuff is now, it's good, far better than what most peeps do.
You want to make it more outstanding, make it bigger, put in even more details. It won't necessarily make it better, but definately more impressive.
I love the oriental buildings, they're downright beautiful.
The small castle(or tower with a wall around it) is very standard. On medieval structures, it pays off to mix all the different stone blocks, stone brick, cobblestone, brick, stone, stair blocks of all the previous types and sometimes sandstone. And wood to accentuate it, like you did with your massive tower, which reminds me of Sauron's Eye, dunno if that was intentional.
There's only one advice I could give and that is to add context to your works. For example, adding terrain and surroundings around your palace, with trees, a path, hedges(leaf blocks on top of eachother) etc. It'll make your buildings stand out more.
I hope to see more stuff, would definately want to see a project if you started on one.
Thanks for the critque guys. About the cobblestone castle, i built that in a day a long time ago, so not that impressive. I will definitely start doing more stuff outside the structures like Smilomaniac said.