Many castelli have been built over time in the fertile valley of the upper reaches of the Mirna River. These forts used to defend this area that has for a long time been the frontier zone between the Venetian Republic and Habsburg Monarchy. Some of these castelli, e.g the partly renovated Castellum Petrapilose, or the completely demolished forts of Crnigrad and Beligrad, have been out of use for a considerable period of time, while others developed into modern towns. This is especially true for Buzet, the central town and the largest community in the northeast of Istria, and Roc, the town that used to be a large Glagolitic centre, and also Hum, known as the smallest town in the world. All bigger ancient forts in Buzet area are presented, and historical development, with town planning and architectural properties, is depicted for all populated communities that have developed at places previously occupied by prehistoric hillforts or medieval castelli.