Reddit user Lycerius has played the same game of Civilization II for almost 10 years. The result? There are only three superpowers left and the world is nothing more than "a hellish nightmare of suffering and devastation." Screenshots are here.
As a result, big cities are a thing of the distant past. Roughly 90% of the worlds population (at it's peak 2000 years ago) has died either from nuclear annihilation or famine caused by the global warming that has left absolutely zero arable land to farm. Engineers (late game worker units) are always busy continuously building roads so that new armies can reach the front lines. Roads that are destroyed the very next turn when the enemy goes. So there isn't any time to clear swamps or clean up the nuclear fallout.