The centre of Argentoratum proper was situated on the Grande Île (Cardo: current Rue du Dôme, 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 Kœnigshoffen. 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 Kœnigshoffen were (found in 1911–12) 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 Église Saint-Étienne 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