The centre of Argentoratum proper was situated on the Grande Ile (Cardo: current Rue du Dome, Decumanus: current Rue des Hallebardes). The outline of the Roman "castrum" is visible in the street pattern in the Grande Ile. Many Roman artifacts have also been found along the current Route des Romains, the road that led to Argentoratum, in the suburb of Knigshoffen. This was where the largest burial places were situated, as well as the densest concentration of civilian dwelling places and commerces next to the camp. Among the most outstanding finds in Knigshoffen were (found in 191112) the fragments of a grand Mithraeum that had been shattered by early Christians in the fourth century. From the fourth century, Strasbourg was the seat of the Bishopric of Strasbourg (made an Archbishopric in 1988). Archaeological excavations below the current Eglise Saint-Etienne in 1948 and 1956 unearthed the apse of a church dating back to the late fourth or early fifth century, considered to be the oldest church in Alsace. It is supposed that this was the first seat of the Roman Catholic Diocese of Strasbourg.
Try to answer this question if possible (otherwise reply "unanswerable"): What was the grand Mithraeum?
unanswerable