It is the end of 1824 made reinforcement of Schützengraben, an arm of the Lotte Bach applied already in the middle ages to the flood.
 After dropping the bottom SOAP Lane, who moved from 1824 until the Ilmufer was the trench "piped" to the library tower visually to the Park to move the surface. To the ILM, the building appears as a bridge, to the library, it is not noticeable as such a structure. If the building is still the old bridge construction or whether it was rebuilt with the reinforcement of the trench, eludes my knowledge. But stylistically, I suspect that it came with the reinforcement to a new building.