I've been watching PotatoMcWhiskey's livestream, and during the Antiquity Era he first lost his capital (and briefly another town) to an Independent Power, admittedly after some gross negligence of the situation (he was under the impression that an IP would never actually take a capital city), and then when he prepared for a war against an AI by moving his troops to the border, the AI reacted by pulling it's own troops into the area, negating the surprise factor that he'd been planning on having, and he almost ended up losing his new capital to that AI before managing to rally his defenses, and did not manage to break through offensively. The only things detracting from the AI's performance are that I feel like it could've taken Potato's capital one or two turns faster with a bit more aggression, and that it offered up the city Potato was after in a peace deal (initiated by the AI) despite having stalled his advances. But all in all, this is a better performance than even the modern Civ VI AI would be able to pull off.