Square.

This is actually exactly what I was thinking about, but in a different architectural style. It doesn't have too much variation in it's pattern, but at the same time it isn't entirely uniform throughout, giving it an interesting look whilst still resembling a fortress.
EDIT: Actually, the use of concrete was used since Roman times for the Pantheon dome(?) so using it in your castle wouldn't be too far fetched (though, it may not retain the style you're going for)
What on earth is going on in the thread.
EDIT 2: P.S. I'm not making fun of your square castle.