Large towns, too, are numerous, and many of them enjoy a deserved reputation amongst the cities of the world on account of their monuments, art treasures, and historical associations. Their number is partly accounted for by the density of the population, and by the facility with which the inhabitants were able to shift their abodes, according to the hazards of war or the vicissitudes of events. And this accounts, too, for the large number of towns which became famous as the capitals of republics, or as royal and ducal residences. Several of the towns at the base of the Alps occupy sites marked out for them by nature. Such are the towns at the mouth of the valleys or defiles, which were places of defence as well as staples of commerce. Ariminum, the modern Rimini, at the southern extremity of the great plain of the Po, was one of these, for during the reign of the Roman it defended the narrow littoral passage between the Adriatic and the Apennines. The Flaminian Road there reached the sea, the Emilian Road thence departed for the north-west, as did also the littoral road of Ravenna. When Rome had ceased to be the capital of the world, and Italy was divided into small hostile states, the towns in the southern part of the plain, or near the passes over the Po, such as Ferrara and Bologna, retained their strategical importance. Piacenza, which defends the passage of the Po between Piemont and Emilia, remains a first-rate fortress to the present day; Alessandria, near the confluence of Tanaro and Bormida, and in a plain famous for many a bloody battle, was likewise destined to become a formidable fortress, though derisively called a “city of straw.” Every valley debouching from France or Austria was locked at its mouth by a strong fort; but most of these places, such as Vinadio, Pinerolo, Fenestrella, and Susa, have become untenable, owing to the range of modern artillery.