Context

This line is spoken by Doctor Faustus in the play Doctor Faustus by Christopher Marlowe. (1594).

Doctor Faustus, MD—that's Magical Dude, not Medical Doctor—summons Lucifer, who gives him an offer he can't refuse. (Lucifer has a lot in common with Don Corleone.) This turns out to be a bad idea, surprising no one. After selling his soul to the devil, Faustus has second thoughts. To convince him that this whole soul-selling thing was a good idea, Mephistopheles, Lucifer's right-hand man, sweeps Faustus around the globe on a wild ride. Some people win trips on game shows. Faustus sells his soul.

During his travels, Faustus meets Helen. Not Helen Mirren, but Helen of Troy, the mythological impetus for the Trojan War. And hot dang, she's a babe. Gazing on her beauty, Faustus understands why thousands of men would go to war because of her.

As for the second part of the quote, the "topless towers of Ilium" wasn't the local gentleman's club. Ilium was another word for Troy, a fortified city with towers so tall the tops couldn't be seen. How will anyone get in? (Sounds like a job for a certain horse.)

Where you've heard it

You've heard this if you're the hottest Helen in the room. Or, you know, during your history lesson on Greek mythology.

Helen had the beauty to launch 1,000 ships, but if you're hot enough to launch just one ship (hey, that's still really hot), your beauty is equal to one millihelen.

Pretentious Factor

If you were to drop this quote at a dinner party, would you get an in-unison "awww" or would everyone roll their eyes and never invite you back? Here it is, on a scale of 1-10.

Not very pretentious, but not recommended as a pick-up line either. Any Helen has probably heard it before, and anyone not named Helen would just be confused.