you could have used the skin modifier or convert to curves and use a bevel.....just ideas lol

for the arches use curves for main shape then another curve to use as a bevel obj then u can have the fancy shapes of the stone work can use a third curve as a taper obj .....big thick base to arch and getting thinner as they get higher and if you don't like drawing curves in blender ..........do drawing in inkscape (other) export as svg and import to blender

hope that's given you some ideas