Yes, 19 cities is quite unusual for my games and at 25 AD it was even 22 cities so I tried to go through my saves looking for an explanation. Here's a list showing the expansion in numbers along with the technologies discovered:
- 2160 BC, 2 cities, TW, Agri, Hunt, AH, Min, BW
- 1800 BC, 4 cities, Pottery, Myst
- 1360 BC, 5 cities, Med, Masonry
- 875 BC, 7 cities, Priest, Writing, Monarchy (Oracle)
- 775 BC, 8 cities, Sail (trade), Arch (trade), Poly
- 675 BC, 9 cities, Alpha (peace deal), IW (peace deal)
- 450 BC, 10 cities
- 350 BC, 11 cities, CoL, Math (trade)
- 225 BC, 12 cities, Currency
- 50 BC, 19 cities, Aesthetics, Philo (bulbed)
- 25 AD, 21 cities
One piece of good luck was that Joao discovered Alpha as the second AI (After Willem) and that made it possible to get this valuable tech in a peace deal. This opened up the possibility to research both CoL and Currency before my capture gold ran out. In addition to this I exploited the marble and got substantial fail gold from building ToA with marble multiplier.This is also the reason behind going Poly, Aesthetics and later Litt. I wanted to open up a lot of marble wonders I could build for massive fail gold thus putting the large empire to good use. With CoL and Currency in hand I was convinced that I could easily turn around the economy using fail gold, courthouses and the additional trade route plus, of course, cottaging the many river tiles. Consequently I pushed hard for expansion from 225 BC until there was no more room for cities.
Another way to look at this is the build stats:
Captured:
8 cities x 100 h = 800 h
3 granaries x 60 h =180 h
The Great Wall = 150 h
10 workers x 60 h = 600 h
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Total of 1730 hammers gained
Lost:
2 axes x 35 h = 70 h
2 archers x 25 h = 50 h
11 immortals x 30 h = 330 h
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Total of 450 hammers lost
The archers and axes plus 1-2 immortals were lost in the defensive wars against de Gaulle and Ragnar. So warring with the immortals against archers was HUGELY favorable. And this estimate is actually conservative because some of the captured cities had more than 1 pop and that should be added as 30 hammers per additional pop. On top I got capture gold and two techs in a peace deal.
I guess my conclusion is that Neil set this up nicely to be very favorable for an early attack with immortals as he hinted in the game description. The combination of the immortals bonus against archers with the nearby placement of an expansive, non-protective civilization really made the difference. If you catch the AI in the expansion phase many cities will be defended by only 1-2 archers and they can be captured easily with a small stack of 2-4 immortals.