The Devil's Tower, one of the 14 remaining medieval towers in San Gimignano, is located on the northernmost side of the Piazza della Cisterna and is part of the Palazzo dei Cortesi.
The tower has a mysterious and unsettling name that is tied to a legend. It is said that upon returning from a trip, the owner noticed with great surprise that the tower was taller than when he left it. This miraculous event was immediately attributed to some diabolical intervention, and so the name of the tower became inextricably linked with the Devil.
The other tower has a double doorway that suggests that there was an ancient foot path passing through it. Right below the tower is the Vicolo dell'Oro, where you find the workshops of the "hammerers, the artisans who vigorously beat gold coin to reduce it to incredibly thin gold leaf, which is then used for painted tables.
Looking up you can notice that the upper floor of the tower has several holes in the walls. These received the timbers that held the galleries (equivalent to our balconies) that allowed people to walk outside, from one room to another. Importantly, these were also able to be rapidly dismantled in case of danger, in order to prevent enemies from accessing the tower.
It is for this same reason that the windows that open on each floor are just narrow slits. This was typical of the Middle Ages, when defence took priority over aesthetics, comfort, and the brightness of the environment.
The top floor would have had a gallery that ran around it as well. This too has only a few supports remaining. Perhaps it was their somewhat sinister appearance that helped fuel the legend.
The Palazzo Cortesi, adjacent to the tower, has only two floors, each one illuminated by a lancet window (a narrow window) and created from a darker stone than the material of the tower.