In 2013 we spent five weeks in Rome. (Highly recommended!) As part of our prep we watched a video series taken from a Yale course on Roman ancient architecture. The professor, Diana Kleiner, teachs a fascinating semester of about 24 sessions. If you will visit any of the forums, the Palatine, the Coliseum, Ostia Antica, Baths of Carracalla, Trajan's Forum, etc, you MUST watch. You only need view a few sessions, as many are outside Rome. We had seen these sites multiple times, but they CAME ALIVE after we saw these classes. We actually re-watched the class on Ostia Antica the evening before our visit, and it changed the experience completely.
Very easy to get. Just Google Yale Roman Architecture to get to the Yale site. It was also available on YouTube last time I checked. This course is like the best college professor you ever had, talking and showing the most interesting subject ever. Just try one. You will be hooked.