The most difficult ideal city in history.

1, recommend Hezhou Fishing City in Sichuan during the Song and Mongolian War. At that time, the Mongolian army set out from East Asia and swept across the Middle East and Eastern Europe. Invincible, but in the fishing city, a Mongolian army fought for decades in the small mountain city of Sichuan. What's more, the death of Mongolian Khan Wokuotai in the fishing city directly led to the suspension of Mongolia's Western Expedition. It was not until decades later that the city was occupied, but it was not shot down by the Mongolian army, but surrendered by the persevering families themselves.

In Europe, I recommend Constantinople, which is now Istanbul, Turkey. The city began to be built during the Eastern Roman period, with mountains and the sea as its back. The terrain was dangerous, but many enemies didn't take it. The Ottoman Empire fought many times without success. Finally, it was taken down with a strange scheme, and it took a lot of effort.