Note i am in no way an architecture master of knowledge. I only talk of what I remember when I did some research projects into the 12th - 14th century eras.
What I know is that half the reason we had the "gothic" style was due to the understanding of how powerful arches were in construction. The art style was, to my knowledge, due to the fact you could now make insides of buildings more "open" or "airy" due to the use of an arch for structure support. The more open the area the more you can "fill it" with better looking stuff.
While I agree the architecture in CoE is non-gothic that doesn't mean the community of builders in the game couldn't just...do such a thing ya know?
My guess for the other half:
Probably something about cost being more effective? Arches being more resistant to siege damage? Better stone molding/plastering? The protruding circles were probably because arches made it easier to structurally build so why not make the circles so you can fire in all directions (just about) when defending.