At first, the ships being small, they could be drawn up on the beach, but as trade increased it was found advisable to build warehouses and sea walls, so that goods could be stored and easily loaded and unloaded. The port having become important, it became vital to protect it from pirates and other enemies, so walls were built about it on the landward side, and sometimes sea walls were built on the water side, in which a narrow entrance was left open during the day and closed with a heavy chain or a floating barricade at night. These sea walls were often as important to shield the ports from storms as to protect them from enemies. Thus the early ports developed, and within these walls were not only all the traders with their goods, but shipyards and those who supplied ships with cordage, lumber, and sails, as well. This simple type of port was the rule down to long after the Middle Ages. As a matter of fact, the great complete structure of the modern port has been developed within very recent times—principally since the introduction of steam. Naturally enough such cities as Venice and Genoa, in their heyday, about or a little after the year 1200, were no longer simple ports, but by comparison with even minor ports of to-day they were simple places. With the development of steam, however, ports became more and more complex. The increased size of ships, the great investments that demanded no loss of time in loading and unloading, the vast increase in the amount of freight and the number of passengers handled—all these, and many other reasons, compelled ports to add complexity to complexity, until the person unfamiliar with the great doings of a busy modern port stands aghast at the vast collection of quays and docks, jetties and sea walls, steam tugs and canal boats, ferryboats and barges, floating grain elevators and great suction dredges, ocean liners and ocean tramps, and a great variety of complicated equipment in the shape of shipyards, coal pockets, factories, warehouses, railroad terminals, and many other things too numerous to mention. [Illustration: A MAP OF THE PORT OF LIVERPOOL _While Liverpool is much smaller, so far as mere area is concerned, than New York, it handles about the same amount of freight. Freight ships load and unload in the tidal basins while passenger steamers use floating landing stages._ ]