[achow]

You are forgetting..

c) Having lots of ships, sailors and powerful army (many in Europe)

d) Being a trading hub (Italy)

e) Having access to lots of valuable metals, salt, gold (richest person in history was Mansa Musa, King of Mali)

[inglor_cz]

Statistical outliers like very rich kings or ports do attract attention, but there was no way to avoid low productivity of agriculture overall. At least 80 per cent of the population had to tend the crops, which lowered the average GDP of entire regions / countries.