The famous cities of the ancient world were surprisingly small and fragile

Even up till 1900 or so, they were very small.

Think of it this way, if you like. Up till around 1900 the world's population was no more than about 1 billion. In that 120 years since then we've grown 8-fold. In other words, where you see 8 people today, in 1900 there was only one. In 1900, every city was only an eighth of its size today (on average).

Many cities had walls. Look at old maps. (Even a map of Paris during the Franco-Prussian War of 1870.) Those walls were in areas that would now be called inner-city, and most of today's cities are now built outside those walls.