What I'd do is put towers every 20 blocks or so, with some circular towers on each corner, and around the front gate. This makes it look more authentic to me.
Oh, and I like the extra smooth stone idea too!
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"I don't care where Leonardo DaVinci got his materials, the Mona Lisa is still a legendary piece of art."
My current wall uses a 4 long by 3 high (or 5x4) section of smooth stone with cobblestone "supports", like so:
I find it to look fairly nice and sturdy, though you can go with other options if you like... a 5-block semi-circle can be a nice protrusion that helps a structure appear more impressive and sturdy, especially if your wall is topped with crenellations or towers or both. My basic wall structure is 3 blocks deep, too, and designed to either contain internal walkways or a padding material like sand, gravel, water, or lava.
Of course, another consideration is that castles were sometimes whitewashed to look clean, bright, and impressive, so building your wall out of wool with an alternate color for horizontal divisions, or possibly having a "painted" icon on the wall, could be good if your main concern is aesthetics.