Yet the difficulties which were ultimately to overwhelm the state were already visible upon the horizon. The eleventh century was a period of nation building in Europe. Poland, Hungary, Bohemia were already coming into existence and aiming for expansion. The Holy Roman Empire, revived under Charlemagne, was encouraging them to turn to the east for their further growth. From the east there came a seemingly endless succession of invading nomad tribes, continuing those movements which had been sweeping over the black earth region for centuries and millennia. The new state had no natural boundaries for defence. Only where the country touched the Carpathian mountains was there any well defined border. In all other directions, south, east and northwest, the land lay open to the invaders. That situation which in times of peace had made Kiev the centre of commerce and had brought it wealth, in time of war was its greatest menace. It was only in the northeast, where the great woods sheltered the primitive Finnic peoples still untouched by culture and Christianity, that there lurked no danger. In all other areas the princes had continually to be on their guard. The danger was greatest to the east, for there they were confronted with the highly mobile nomad troops who could attack with startling suddenness, ravage the country, and if necessary disappear with the same speed. The heart of the state was the line of the Dnyeper and so long as that was not cut, it was possible for Kiev to exist in relative security. Outside of that, there were scattered throughout the land various lesser cities, such as Chernihiv and others, which served as rallying points for the princes and their forces. If it were possible to coordinate these into an effective system, all would be well.