At ItalianSide, we have a big collection of old photos from almost every town in Toscana and in Italy (We have more than 90% of Italian towns in our pictures archives!).
We have several images of Castel San Niccolò too and here you can see one of them (click on the picture to enlarge).

If you are interested in historical maps, here you can see an old map (1800) of Castel San Niccolò and its area.

If you are interested in this or other pictures with no watermark of Castel San Niccolò just send an email to castelsanniccolo@italianside.com, with your request.