Tram lines passed through the middle of the Lodz ghetto. Instead of changing the route of the trams, the Germans isolated them with fences, and policemen patrolled them the area in order to prevent the Jews from escaping in the trams.
Most of the ghettos were sealed with barbed wire or walls, and the entrances were guarded but local and German policemen, and also by members of the SS. Many of the ghettos were created in cities that already had a large concentration of Jews. The Germans generally chose the oldest and degraded district in the city and there they sealed the ghetto. Most of the times it was necessary to remove the non-Jewish population from those areas so that the Jews could be deported.
Millions of Jews lived in Eastern Germany. After Germany invaded Poland in 1939, more than 2 million Jews became subject to German control. The ghettos had a key role in the Nazi strategy of controlling the Jewish population. More than 1,000 ghettos were created in the occupied territories.