The Fall of Babel - Josiah Bancroft

As Marat's siege engine bores through the Tower, Senlin can do nothing but observe the mayhem from inside the belly of the beast. Edith and her crew are forced to face Marat on unequal footing, with Senlin caught in the crossfire, while Adam attempts to unravel the mystery of his fame inside the crowning ringdom. And when the Brick Layer's true ambition is revealed, neither the Tower nor its inhabitants will ever be the same again.