How to inset odd shape Disclaimer: Fairly new to 3D modeling; done some simple things, trying to model stuff around me.

I am trying to model a space where I live (Gibraltar in case you care) that is essentially a repetition of the sections pictured below. It looks like the arch could be done as an inset into a larger face, but how to I set up to inset an odd shape like that? Is this even the best approach or would I be better off building it up from smaller chunks?
You should begin with the circle arc, then extrude up, down and inwards. When you'll have finished, use a texture to create the stones.