Finally, the real iron containment by means of engineer equipment was built.[1] From 1951 onwards, this area was guarded by a signal fence some kilometres inside the border, while the border itself was secured by a guarded strip with a single barbed wire fence. This fence, originally an electric fence with a voltage of 5000 V, was replaced, starting in 1968, by a double wire mesh fence similar to that used on the Inner German border. In addition, the border was fitted with watch towers.