Medieval technology? The middle ages invented, among other things, the crank, the horse collar, eyeglasses, the flying buttress, the stirrup, the windmill, the wheelbarrow, printing, firearms, paper, the canal lock, the compass, the rudder, the mechanical clock, the spinning wheel, and the treadle.
"Ahu" are stone platforms on which many moai sit. There are 313 known ahu and 125 of these carry moai. The biggest, Ahu Tongariki is 220 m (720 ft), and had the most (15) and tallest moai.
