SETTLERS build IRRIGATION on land beside fresh water — a RIVER, LAKE, or an irrigated tile.
Irrigation adds +1 FOOD to the tile, feeding a larger city.
Not on forest, mountains, or the open ocean.
Food is a city's lifeblood: more food means faster growth and more citizens to work tiles.
Chain irrigation inland from a river or lake to green whole provinces.
DESPOTISM docks tiles giving over 2 food; change government to lift the penalty.