In the fertile region between the Danube and the Balkans occur numerous Byzantine fortified posts, which are shown by occasional finds of coins to date mostly from the sixth century. All have a common form in that skilful use has been made of natural features of terrain, such as isolated mountain-tops or practically isolated spurs, so that with a minimum cf artificial strengthening they have considerable defensive power. A favourite situation for them is at points where narrow canyon-like valleys of the streams that flow into the Danube have afforded good natural advantages.
From the military standpoint these sites can easily be divided into two groups, A and B, according to their topographical situation. To the first belong those which occur on the great natural trade-routes, and, like a modern Sperrfort, rake the roads with a commanding view over the surrounding district. The second and much more numerous group consists of posts situated off the trade-routes, often so much concealed by the lie of the land as to be invisible from the great roads.