'O fortunati, quorum iam moenia surgunt!'
Aeneas ait, et fastigia suspicit urbis.
Vergil, Aeneid, I: 437-438.
"O fortunate ones, whose walls now rise up!"
Aeneas says, and he looks up at the rooftops of the city.
(iam is the adverb for now, already: it denotes a temporal urgency, a sort of presence of being)
Aeneas is embittered, grief-stricken, perhaps even a little jealous of the Carthaginians. His ships and his men have been torn asunder by the winds of Aeolus, the ire of Juno; only the intervention of Neptune, who halted the storm and calmed the seas, saved Aeneas and his men from certain death. Worse, right now many of his ships and his men are missing. It is incontrovertibly prophesied that Aeneas will one day found an exalted city, but he knows only where (Hesperia, or Italy, his 'promised land') and not with whom, or when, or how this glorious city-state will be founded.
Having landed, Aeneas has visually inspected his surroundings from a rock; alas, there has been no sign of his lost companions, but he has managed to catch and kill practically a whole herd of stags for dinner and has slept, pensive and contemplative. This morning, he has explored the area more thoroughly with his faithful companion Achates and has come across the city of Carthage, still under construction. Seeing the very many men building the foundations of what looks to be a magnificent city, Aeneas is reminded of his own destiny, which in this foreign land seems so very distant, so unattainable. Here, then, Aeneas is very much the Vergilian hero, pious but self-doubting, aware of his fate and his duty, cognizant of the terrific onus he carries.
And so he looks up at the tall pillars, the vast columns which are only the groundwork laid for still greater future things, at the rooftops beneath which hundreds of thousands of happy citizens will one day reside, and (sadly, but truthfully) from his lips escapes that famous epithet:
"O, fortunate ones, whose walls already are rising up!"
This line reflects many of the qualities we learn about Aeneas in Vergil's epic. It is a simple, direct statement of Aeneas' deeply-held desire to fulfill his obligation to his people by founding a city of his own; it is a clear expression of his distaste for homelessness, for (having fled the burning ruins of his native Troy) he wants somewhere to belong again.
Poetically speaking, check out the -i assonance; it emphasizes Aeneas's cry. Metrically speaking, the four strong spondees, followed by a dactyl and another spondee (these last two are typical of almost every line of the Aeneid), give a sense of power to what Aeneas cries out. After he speaks, the early caesura (that comma-delineated pause) shows that he is more than normally pensive as he reflects upon the city before him; again, the strong spondees in "ait, et fastigia" show his inner conviction and forcefulness. These lines are among the more famous in this book of the Aeneid, and rightfully so; like so much of what Vergil wrote, they are a masterpiece of Latin poetry and of literature as well.