What is the allusion given in poem Romeo and Juliet which mentions a famous place?

The most famous allusion to a place in Romeo and Juliet is "a plague o' both your houses!" This curse, uttered by Mercutio in Act III, Scene i, alludes to the ancient city of Ephesus, known for its many plagues and pestilences.

While not directly named, the reference to Ephesus is clear through the context of Mercutio's speech. He is dying, lamenting the feud between the Montagues and Capulets, and cursing both families for their destructive hatred. The association of Ephesus with plagues reinforces the notion of the destructive power of the feud, as though it were a deadly disease.

This allusion adds another layer of meaning to the play, highlighting the destructive nature of hatred and the consequences of feuding.