Round, in a square world

Someone must have made a decision to put it there at one time. The barn is old, probably late 18hundred something. It's not that the building material is scarce, this island consists of limestone, easily cut up into square building blocks. So, why put a round rock in the wall? It must have meant a lot of extra time, work and consideration and it doesn't seem to have any particular function. 

The rock is put exactly under the hatch, probably for aesthetic reasons and it has not been plastered so it's there to be noticed. Maybe it has even been painted in a yellowish color, it's hard to tell. Did the builder have a sense of humour or simply wanted to break the tedious work of putting square rocks on top of each other, or did he want to be remembered. I will never get an answer to any of my questions but it affects me, putting a smile on my face every time I pass by. 
Maybe the builder foresaw that.