After Sigmaringen is left behind the valley is less narrow, and the river goes on past small villages and old towns, each of which has no doubt its interest for the leisurely pedestrian. Beyond Reidlingen on the left bank is seen on the right the isolated hill of Bussen, from the summit of which is to be had a view embracing the whole of Upper Swabia and much of the Alps. On the hill is a pilgrimage church and a ruined castle The ruined castles are so numerous along parts of the river that is not possible to pause at all; for it would appear as though in the good old times the population largely consisted of castle-dwelling barons. Zweifaltendorf has a stalactite cave; Rechtenstein, with another ruined castle, is a notably beautiful spot; Ober-Marchtal has a grand old Premonstratensian monastery; while Munderkirchen, built upon a rock islanded by the forking river, has a new stone bridge with an arch span of one hundred and sixty-four feet. Beyond this the valley is wider. At the village of Ehingen, the railway turns northward from the river, which between here and Ulm receives several affluents from the south, including just before Ulm the Iller, while at Ulm itself, the river Blau comes in from a delightfully wooded and rocky valley on the left. Ulm, the frontier town of Würtemburg, is important in the story of the Danube for a variety of reasons. In the first place, it is here, fourteen hundred feet above sea-level, that the river becomes effectively navigable for flat-bottomed boats of about a hundred tons, and thus it is the centre of a brisk trade. Then it is a picturesque old city, with many ancient houses still to show, and it was long regarded as a strategic point of great importance, and, therefore, was maintained as a fortress of first rank. It was said, some years ago, that it was capable of sheltering within its fortifications a force of a hundred thousand men. Latterly, it has developed as an industrial and commercial centre, and the ramparts have been acquired by the town for peaceful purposes.