Towards the end of book 2, Games of Rome, Gaius along with Max, Bryaxis, and Simon, walk through the construction site of what will become the Forum of Trajan in Rome. At one point they come upon the partially erected Column of Trajan. And Bry snarkiness ensues.
Here’s a lovely article on the 100 foot, marble column of Trajan with an internal spiral staircase, looking from the inside out. Great stuff. http://floorjournal.com/2013/06/23/trajans-hollow/