Unfortunatly, I erased my castle design. Let me tell you that I know a bit about this subject. You can do it really nicely if you create a small "mine entrance" and mine a series of winding corridors. You get a nice "dungeon" effect over there and get plenty of material for the castle construction. You can later connect the mines to the castle to make an "escape" hatch. It also makes a good place to set wicked traps and place your best artifacts.
This is an impressive one, but I see some problems. Look at the towers, appearing to "hover" in the air. Annoying.
Another thing would be the tower setup.
I prefer 2 wide walls in my castle walls, with the outer ones ending in fortifications. Also, towers should have fortifications in them so to provide places to fire out of.
Here is a castle that I am building.
http://mkv25.net/dfma/map-3142-playpapers-harracothIt is quite an efficient design as everything underground is aligned along 9 staircases. The workshops, dinning rooms, stockpiles and bedrooms are all close to each other yet far enough away to stop noise pollution.
Only 1 Z-level. It's not an impressive castle. I feel walls should be at least 2 Z-levels, towers should extend at least 2 beyond the hight of the walls. Really the best tactic is to buckle down and make one.