The only places of exit from these great palisaded fortifications were very narrow gates, or sally-ports, opening at proper intervals, and well guarded by armed sentinels. The space between the successive ramparts was a well-wooded and thickly-settled country, filled with villages and homesteads, so close together that the sound of a trumpet could be heard from one to the other, and thus an alarm from the exterior be conveyed with remarkable rapidity throughout the whole land. This and more the veracious Monk of St. Gall tells us. As to believing him, that is quite another matter. Sufficient is told by other writers to convince us that the country was guarded by strong and singular defences, but the nine concentric circles of breastworks, surpassing the Chinese wall in length and size, the reader is quite privileged to doubt. Certainly the defences failed to check the advance of the army of Charlemagne. Though he had begun his march in the spring, so extensive were his preparations that it was September before he reached the banks of the river Enns, the border line between Bavaria and Hungary. Here the army encamped for three days, engaged in prayers for victory, and here encouraging news came to Charlemagne. His son Pepin, with the Duke of Friuli, had already invaded Hungary, met an army of the Avars, and defeated it with great slaughter. The news of this success must have invigorated the army under Charlemagne. Breaking camp, they invaded the country of the Avars, advancing with the usual impetuosity of their great leader. One after another the Hungarian lines of defence were taken, until three had fallen, while the country between them was laid waste. No army appeared in the path of the invaders; sword in hand, Charlemagne assailed and broke through the strong walls of his foes; soon he reached the river Raab, which he followed to its junction with the Danube. Until now all had promised complete success. Those frightful Huns, who had so long kept Europe in terror, seemed about to be subdued and made subjects of the great monarch of the Franks. But, through that fatality which so often ruins the best-laid plans of men, Charlemagne suddenly found himself in a perilous and critical situation. His army was composed almost wholly of cavalry. As he lay encamped by the Danube, a deadly pestilence attacked the horses, and swept them off with such rapidity that a hasty retreat became necessary. Nine-tenths of the horses had perished before the retiring army reached Bavaria. Good fortune, however, attended the retreat. Had the Avars recovered from the panic into which their successive defeats had thrown them, they might have taken a disastrous revenge upon the invaders. But as it was, Charlemagne succeeded in retiring without being attacked, and was able to take with him the valuable booty and the host of prisoners which were the trophies of his victorious progress.