It's not on the list of biggest cities in the world now, but archeologists have determined that the city of Angkor, which is now ruins in Cambodia, was the world's largest preindustrial city. Covering 390 square miles (1,010 square kilometers), Angkor was centered on the temple of Angkor Wat, one of the largest Hindu temples in the world. Though it was once a bustling metropolis, much of Angkor now lies in jungle. The United Nations Educational, Scientific and Cultural Organization (UNESCO) names Angkor a World Heritage site in 1992, which works to protect Angkor.
Because it was the seat of the Khmer empire, Angkor is home to many other sites, including the Terrace of Elephants, palaces, and the Bayon, a Buddhist temple. The area around Angkor is fairly rural, and you can see traditional farming methods, as well as learn about the impact of land-mines and other artifacts of more recent history, on the people and the area.
Though much of it has been reclaimed by jungle, Angkor remains a popular and educational tourist destination. If you decide to visit, you'll learn a lot, but you should also keep in mind that these are still religious sites. You'll want to dress and behave respectfully. Angkor is also an immensely important archeological site. Though parts of "Tomb Raider" were filmed in the area, you're not Lara Croft. Don't take anything but pictures.