Symbolism, Imagery, Allegory
In Freud's multiple Dreams of Rome, Rome itself has many symbolic meanings.
For one thing, as one Freud fan points out, it's "an object of both enmity and desire" that Freud associates with the Roman Catholic Church, which "represented for him the stronghold of religious obscurantism in contrast to his own scientific enlightenment" (source). For another, it's also a symbol of "the anti-Semitism which was indeed rife, often with clerical support, in Freud's Vienna" (source).
Not too tricky, right? Let's take a closer look at one of Freud's Dreams of Rome to see how other kinds of imagery and symbolism combine to give "Rome" its symbolic meaning.
In one of his Dreams of Rome, Freud sees "a narrow stream of dark water; on one side of it were black cliffs and on the other meadows with big white flowers" (5.3.9). He also sees "a Herr Zucker […] and determined to ask him the way to the city" (5.3.9).
Freud's interpretation of this dream's symbols goes a little something like this.
White flowers: These flowers symbolize Ravenna, a city that Freud had actually visited, and "which, for a time at least, superseded Rome as capital of Italy" (5.3.9). Since Freud was unable to visit Rome before 1901, his dream uses the white flowers to symbolize a city that he had visited and that replaces Rome to some extent in the dream.
Black cliffs and Herr Zucker: Freud notes that "[t]he dark cliff, so close to the water" reminded him of "the valley of the Tepl near Karlsbad" (5.3.9). Once he makes this connection, Freud realizes that the name "Karlsbad" reminds him of two Jewish anecdotes that also explain the significance of Herr Zucker in the dream. These two symbolic elements come together to signify the phrase "Asking the way," which Freud interprets to be "a direct allusion to Rome, since it is well known that all roads lead there" (5.3.9).
As in Freud's Dream of Irma's Injection, in this Dream of Rome, there are very few "universal" or "common" symbols. Instead, the symbolic meanings of the dream-elements are unique to Freud's own experiences and associations.