Water mills have a horizontal wheel, called a caster. This device, composed of a set of radially exposed blades that receive the impulse that the water transmits when hitting them.

This type of system spread across Europe at an accelerated rate due to the characteristics of water courses and the increase in cereal cultivation.

In Portugal, it is believed that the introduction of water mills was made by the Romans. In the northern region of Portugal it was the most adopted system, and in 1968 there were around 10,000 mills in operation.