By the end of the 14th century, most rural towns had an average of up to seven mills. Water-powered mechanisms operated hammers, fulling mills, sawmills, water pumps, bellows (in forges and foundries), or devices for breaking hemp.
Each mill consisted of three parts: the motor, the transmission system and the working device. The motor was the water wheel, and the transmission system comprised various components that changed the direction of force, ensuring a smooth transition from the slow rotation of the water wheel to a faster one.
The working device varied according to the purpose of the structure. The core of grain mills were the millstones: the stationary lower stone called the bedstone and the rotating upper stone called the runner. Grain was ground between the stones.
Proper placement was crucial for a mill: the current had to be neither too weak nor too strong, and in our climate, the strength of the river current varied significantly with the seasons. Therefore, the miller also had to build artificial channels and weirs to regulate the flow.
— West of the Vltava, close to Charles Bridge in Prague, lies Kampa Island, separated from the rest of Lesser Town by the so-called Čertovka, the Devil's Canal. The oldest records of both Kampa and the canal date back to the 12th century, when Čertovka – known in the Middle Ages as Rožmberská strouha, or Rosenberg Ditch – was dug by the Knights of Malta to power a row of nine watermills along its path, and the various fields, gardens and vineyards that belonged to the Knights and were situated on Kampa Island. Of the nine watermills, only three remain intact, with the best preserved one being the Velkopřevorský Mill, the Grand Priory Mill.
Next to its still rotating wheel, the Grand Priory Mill is also equipped with a figurine sitting close by: a little green man with a round belly, a hat made of reed and a pipe in his hand, a so-called VodnĂk. Legends of this waterman are recorded since the 16th century, and likely told since pre-Christian times throughout the entire Slavic world, as VodnĂk in Czechia, Povodni Moz in Slovenia or Topielec in Poland, and even the water nymph in Grimm's fairytale "The Nixie of the Mill-Pond" leads back to the same roots. The VodnĂk would live in marshes, ponds, rivers or lakes and, most typically, inside watermills on whose wheels he would sit after sunset. As a spirit, he could appear both benevolent, in which he might help fishermen with their catch or increase the local harvest, or malevolent, scaring off fish, damaging dams and mills or drowning people. Sometimes, even the overflowing of rivers was attributed to the VodnĂk, though in this case, he wouldn't do it on purpose – it would rather be a by-product of a huge underwater wedding party the VodnĂk was throwing.