The concentric circles is a good choice for a square crop. But I'm wondering why you chose small dimensions and such a huge border. That leaves us with so much less photo area that it's hard to see the details.
This is essentially what I think of when I think of the perfect subject for a square crop: Symmetry. I took a similar shot that I would have loved to have entered but I took it the day before the challenge was announced and I had no chance of getting back to it. Nice graduated tones, well framed. Great find.
A bit heavy on the border. I do love our rotunda though. I've been known to lie on my back amidst many tourist and DPC officers to shoot this exact image. :)