"Great Engineers" are the men and women who get things done. They construct the monuments, the roads, the information networks that improve the lives of those around them. They invent new processes for building high-rises or rolling steel; they dig canals; they irrigate deserts. They keep a civilization moving forward.
Great Engineers can construct the special Manufactory improvement, which, when worked, provides a lot of Production for a city. The Great Engineer can also hurry production of a unit, building or Wonder in a city. The Great Engineer is expended when used in either of these ways.