From what you have read you will readily understand that the Vosges are a formidable barrier to invasion from the east. To the west of the main chain you see another ridge of heights, and beyond them the valley of the Upper Moselle. On this river, not far from its head-waters, is the second great barrier fortress of France--Epinal.[135] To the north of Epinal, and about ten miles west of Nancy, is the third fortress--Toul.[136] The fort of St. Michel, about twenty miles north-west of Toul, is the key to the circle of forts that defend the entrenched camp, and the strongest fort on the frontier. If you were to visit Toul you would see little or nothing of the batteries, for they are hidden in brushwood and stunted woods high above the vineyards. Farther to the north, about thirty-five miles west of Metz, is Verdun, which has already been mentioned in these pages. As Verdun is the only barrier fortress which was seriously attacked by the Germans during 1914, let us learn something of its story. Verdun is a great entrenched camp, contained within a ring which measures thirty miles round. There are sixteen large forts and about twenty smaller forts on this ring, and the most distant of them is about nine miles from the centre of the city. All these defences have been constructed since the Franco-German War, during which the city was bombarded on three different occasions. It yielded early in November 1870. During the Battle of the Marne the Crown Prince made a great effort to capture Verdun. I have already told you that he battered down Fort Troyon,[137] but was unable to capture it, though it lay in ruins. Between the 10th and 12th of September the Crown Prince's army, along with the other German armies, was forced to retreat. It fell back two days' march to the north, and immediately the French general, Sarrail,[138] prepared Verdun to stand a long siege. Seven thousand civilians--"useless mouths," as the French soldiers call them--were ordered to withdraw, and the food supply for the garrison was regulated.