Wonderful work Lambert.
The lighting looks very convincing and realistic to me. Love the atmosphere you created.
So don't worry about the cornerstones. This is the way the array modifier works, makes a duplicate of the object and the material.
Sorry for my late reply Chris but I had a nasty little cold and therefore I wasn't very active.
About those corner stones; I could have joined them together making one object and use the boolean modifier to intersect with the cube I used to make the 'gate'.
Then I could have reduced a very tiny bit the size of those corner stones and position them so that a very small gap was visible between them and the gate.
Now it looks a bit too slick.
I used 2 light sources, the actual lamp and the background set in faint emission making the light visible on the door post and the cobble stones right behind them.
But actually the gate/crypt was still too dark with difficult to see details so I added an exit sign glowing red to illuminate that crypt.