View larger size Weir and Locks In the early 1920s, the Neckar River was turned into an efficient water route be-tween Mannheim and Heilbronn. To achieve this, it was necessary to build a total of 18 weirs with locks to permit watercraft to pass – one of them opposite Karl’s Gate. It was planned and built to take into consideration the special situation there at a very narrow point of the Neckar Valley near the Old Town. The walkway over the river was installed upriver and the high piers downriver, and it was all covered with red sandstone so it would blend in with the surrounding houses. The other Neckar weirs were equipped immediately with hydroelectric power plants, but for a long time this could not be done at Karl’s Gate because the difference in water levels was too small. It was not until 1998 that an underwater generator was also installed there to produce electricity.