15thc perception on swallowtail merlons?

[Koen G]

Especially for the Annunciation, there's great symbolism tied to Mary's surroundings, and everything is about her virginity. Walled garden, sanctity of her bedchambers, the pillars acting like a barrier between her and the angel, God's rays passing through the window... It's almost obligatory. 

The crucifixion is set outside the city walls, because that's where the bible said it took place. You're still near Jerusalem, Christ has been pushed out. 

Before modern times, the expectation was that images must translate ideas, thoughts and feelings in a way the audience can relate to. In Flemish art, you might see the skyline of Bruges or whatever, to show: Jesus was in our city, but we (humanity) expelled him and killed him.

In that sense, I think the merlons in Italian frescos are there because either the real or archetypal city had merlons in the audience's mind. So when we see swallowtail merlons there, I would assume "this is what the patrons expected a city to look like".