15thc perception on swallowtail merlons?

[Koen G]

That's remarkable indeed. In manuscripts, many of the merlons we find are also in places in the Holy Land. Did they have some exotic appeal, even for Italians?

Now on the other hand, it may just be that frescos are predominantly of religious scenes, so you're bound to get lots of merlons in religious urban settings? (Lots of the scenes you mentioned are set in/against Jerusalem, which is important to include in imagery).