It's not just that it's one of the oldest cities, it's arguably the oldest "major city" that we know of. Around 8,000 B.C. Jericho is thought to have had upwards of 3,000 residents at a time when the total human population on earth is estimated to have been around 5,000,000. Which means this city had twice the percentage of total human population living in it that New York City does today with an 8.5 million person population (about 0.03%). Even more importantly this city had a wall. Not a simple palisade either. A stone wall over ten feet tall and almost six feet thick at the base. There were also stone towers built into the wall; one of which was almost 30 feet high. there is also a wide ditch dug all the way around the town's walls that goes all the way into the bedrock and was almost certainly defensive in nature as well. No other city from this time period or before that we know of had defenses anywhere near this strong. It would have taken a very large, prosperous and organized population for the time to not only make having such defenses worth it, but to be able to build them as well.