London is a port on the Thames (see main article "Port of London"), a navigable river. The river has had a major influence on the development of the city. London began on the Thames' north bank and for a long time the main focus of the city remained on the north side of the Thames. For many centuries London Bridge was the only bridge in or close to the city. When more bridges were built in the 18th century, the city expanded in all directions as the mostly flat or gently rolling countryside presented no obstacle to growth.