Hi
OK, the opening does not work at all - either You make an arch and clearly mark it out like in the picture or You have it straight and then a one-piece slab would be there - otherwise it would collapse
the three piece effect - not necessarily a problem
irregular stonework like You chose to represent always has levelling inserts of same size irregular or regular stones or bricks. This is necessary to even out the pressure of the weight
so if You simply break it up again and insert two regular lines it's done.
Also usual but not necessary is the lowest level made by bigger and sometimes more regular slabs, again to even out the pressure
But these can sometimes be buried in the ground and thus not visible
the arch You can do by simply covering the curved section with soft cardboard - archwork occasionaly protrudes beyond the wall surface - don't forget the big cornerstones
if You want to keep it generic, no need to spend more thoughts on it - if You want to do it especially for a certain period, things get more complicated.
The medieval wall You did is accurate, however it would have been covered with plaster and painted. Thus You could add some leftover plaster patches of an untended wall. Colour chalkish white or clay yellowish (generic)
OK, enough
