Ancient Roman mills were used mainly to pound grain and grind flour but also to crush ore, saw wood and stones. These mills used human, animal, and water. The mola asinaria which dates back 200-300 B.C. was a basic rotary mill driven by blindfolded horses, donkeys or mules. Sometimes it would use human power and be driven by slaves. It had a horizontal axle attached to a shaft which was attached to a runner stone (the upper stone). There would be a bedstone (the lower stone) below the runner stone stone usually with a conical shape which was stationary. The stone used was often lava because of its strength. The mola asinaria was mainly used to grind flour and corn.
The watermill was developed at the beginning of the Empire. There were various kinds of watermills using different kinds of water wheels. There were three kinds of vertical water wheels including the undershot water wheel (water hitting the bottom of the wheel), the breastshot water wheel (water hitting the middle of the wheel) and the overshot water wheel (water hitting the paddles at the top of the wheel). The overshot water wheel was the most efficient and generated the most power, while the undershot water wheel which usually used the stream of a river generated the least power. There was also the horizontal water wheel. Often the watermills would use the water power from a nearby aqueduct. Water from the aqueduct would be redirected to the water wheels at high speeds through a system of tanks and pipes.
These mills were built at the end of the 1st century A.D. and operated until the end of the 3rd century. It is believed that hundreds of people used to work at the complex and that the complex produced enough flour per day to supply 20,000 to 40,000 inhabitants. It is worth noting that the Barbegal complex used overshot water wheels which also generated the most power.
Watermills were mostly used to grind cereals but also to crush ore. Many mines had watermills and aqueducts built nearby. The aqueducts could be very long and could bring water from far away rivers. The water was collected in large tanks. The miners would locate veins of minerals on the surface and then unleash a powerful flow of water from the tanks in a method called hushing. The powerful flow of water could also be used to power watermills. For example, it is believed that the Dolaucothi mines had water powered mills nearby to crush the ore collected by the miners.