Built on three levels, Emperor Trajan’s Market housed 150 shops and commercial offices—think of it as the world’s first shopping mall. Grooves still evident in the thresholds allowed merchants to slide doors shut and lock up for the night. You’re likely to have the covered, tunnel-like market halls mostly to yourself—making the ancient past feel all the more present in this overlooked site. The Museum of the Imperial Forums occupies a converted section of the market, and has excellent visual displays that help you imagine what these grand public squares and temples used to look like. All in all, it’s home to 172 marble fragments from the Fori Imperiali; here are also original remnants from the Forum of Augustus and Forum of Nerva.