One of the niftiest things I saw in Germany while I was doing research for Elisha Mancer were the Kranen of Trier–the medieval cranes used to unload boats on the nearby Mosel River. I was fascinated by these curious round towers on the riverside. The wear pattern around the mooring rings indicated many years of use–boats tying up to make use of the cranes.
Like a windmill, the top of the crane can rotate to aim the lift arm more effectively and guide the crates from the boat to the shore. Inside the crane is a huge wooden wheel apparatus that operates the pulleys and rotates the arm. The crane is human-powered: like a giant hamster wheel. The technical term for these cranes is the Treadwheel Crane. Wikipedia has some cool images from art and illuminations, as well as reconstructions of treadwheel cranes from the Roman era.
Small windows in the base allow for communication and visual confirmation with the operators. Yes, these cranes have two wheels to allow for faster unloading with two people working at once.
One of my frustrations with pseudo-medieval fantasy is that they often feature technologically stagnant societies, as if human ingenuity would simply cease with access to magic. Now, there are some books in which magic can, apparently, do anything without limits or costs (a trope which irritates me no end), but in most cases, the magic is limited to certain individuals or certain kinds of tasks, and clearly has a higher cost than simply having ordinary mortals doing their jobs. And, as mortals, do, seeking ways to make their jobs easier.