After escaping the plummeting zeppelin and taking another trip on the Skylines, Booker finally arrives at the Monument Tower. It's a positively enormous statue of Columbia, the Lady Liberty-like angel who's supposedly Comstock's heavenly benefactor. Somehow we doubt there's anything heavenly about Columbia at all, but that's another story.
The entire tower is locked down, with signs everywhere warning of the danger of the being who's locked up at its apex. Booker presses on despite the apparent danger, only to find that it's Elizabeth herself at the top. She's been locked there her entire life, watched by researchers attempting to discover…something. We don't know. But she is considered the sacred spawn of Father Comstock, granted immense power and destined to take his place when he dies (presumably of cancer—there's mention of tumors).
You'll get the chance to watch Elizabeth, and besides opening "tears" in space-time to other realities, she's a normal girl—loves dancing, Paris. You know the type. Long story short, she wants to leave as much as Booker does; but obviously, things aren't going to be that easy, and the deadly mechanical Songbird shows up and wrecks the entire tower as Booker and Liz try to escape. The scene ends with a plummet down into the water below and a too-close look at the Songbird's inhuman eyes.