According to the Oxford English Dictionary, the earliest known use in English of the word "balcone" (as it was then spelled) didn’t occur until 1618, two years after Shakespeare died. Even the concept of a balcony was (literally) foreign to Shakespeare's British contemporaries. In 1611, more than a decade and a half after Romeo and Juliet was first performed, an Englishman named Tom Coryat published an account of the tour of the Continent he undertook in 1608. His whopper of a title, Coryat's Crudities: Hastily Gobled Up in Five Moneths Travells in France, Savoy, Italy, Rhetia Commonly Called the Grisons Country, Helvetia Alias Switzerland, Some Parts of High Germany and the Netherlands: Newly Digested in the Hungry Aire of Odcombe in the County of Somerset, and Now Dispersed to the Nourishment of the Travelling Members of this Kingdome, indicates how exotic and unknown he presumed the nations he visited were to his English readers. Italy, which figures prominently in many of Shakespeare's plays, was a source of especial architectural fascination for Coryat, even without the word "balcone" to describe what he saw:
I noted another thing in these Venetian Palaces ... and it is very little used in any other country that I could perceive in my travels, saving only in Venice and other Italian Cities. Some what above the middle of the front of the building, or ... a little beneath the toppe of the front they have right opposite unto their windows, a very pleasant little tarrasse, that jutteth or butteth out from the maine building: the edge whereof is decked with many prety litle turned pillers, either of marble or free stone to leane over. These kind of tarrasses or little galleries of pleasure ... serve only for this purpose, that people may from that place as from a most delectable prospect contemplate and view the parts of the City round about them.
For 17th-century English readers, there was something nearly scandalous in what Coryat describes, because the bodies on these jutting, butting galleries of pleasure now known as balconies weren't just viewing; they were also on view. As Henry Wotton, another Englishman, put it in his 1624 treatise The Elements of Architecture, there is "in no Habitations lesse privacie" than those of the Italians. The strangeness of this architectural feature thus stood in for larger national and cultural differences: Shakespeare's England was too cool for such architectural innovations, in terms of climate (they were experiencing a mini-Ice Age) but also perhaps in terms of social or sexual temperament.
So how did the culturally charged image of the balcony become so closely associated with Romeo and Juliet that it now serves as a visual synecdoche for the play itself?
The staged scene most strongly associated with Shakespeare actually comes from another playwright entirely, Thomas Otway. Little known today, Otway serves as a reminder that a famous playwright and exceedingly popular plays can fall out of public favor—as happened to Shakespeare, and particularly to Romeo and Juliet, which for nearly a century was rarely performed. In 1642, the Puritan Parliament, at war with King Charles I, closed London's theaters. After Charles II was restored to the throne in 1660 and the theaters were reopened, Shakespeare plays were put on again, including a 1662 revival of Romeo and Juliet. But far more popular was Otway's 1679 play, The History and Fall of Caius Marius, which grafts dialogue, characters, and plot from Romeo and Juliet onto an ancient Roman military and political struggle drawn from Plutarch. Although Shakespeare himself often borrowed heavily from a wide range of sources, Otway's own substantial appropriations—as when the young heroine Lavinia soliloquizes "O Marius, Marius! wherefore art thou Marius?"—might strike modern audiences as a nearly sacrilegious level of plagiarism.