When the war broke out, the cities were the armed guardians of German trade, which was carried on with wealth and bustle, in narrow streets between high houses. Almost every city, with the exception of the smallest market towns, was shut out from the open country by walls, gates, and moats. The approaches were narrow and easy to defend; there were often double walls, and in many cases the old towers still overtopped the battlements and gates. Many of the more important of these middle-age fortifications had been strengthened in the course of the century, the bastions of stone and brick-work, as well as strong single towers, were mounted with heavy artillery; and frequently the old castle of some landed proprietor, or the house of some former magistrate or count appointed by the Emperor, were fortified. They were not fortresses in our sense, but they could, if the walls were thick and the citizens stanch, resist even a great army, at least for a long time. Thus Nördlingen maintained itself in 1634 for eighteen days, against the united Imperial armies of King Ferdinand, Gallas, and Piccolomini--forming together more than 60,000 men: the citizens repulsed seven assaults, with only five hundred men, Swedish auxiliaries. For a defence like this, earth sconces were thrown out as outworks, and rapidly united by trenches and palisades. Many places, however, far more than at present, were real fortresses. Their chief strength consisted in their outworks, which were planned by Flemish science. It had long been known that the balls of carronades were more destructive to stone and breast-works than to earth-works.