Symbolism, Imagery, Allegory
Okay, so you've got two young lovers whose families won't let them anywhere near each other. The only way they can communicate is through a crack in the wall that separates their neighboring houses. Is it just us, or does the wall that separates them seem like a pretty blatant symbol of the families that are keeping them apart?
It's a little ironic, though, because the wall that separates these two households is also one of the few things they share. Though the families hate each other, there's not much they can do about the fact that their houses share a wall and that their children share an immense affection for each other.
That means that the wall can be seen as a symbol of the hatred that divides Pyramus and Thisbe as well as the love that brings them together. Fancy.