challenge: What was the primary purpose of building the Irgenhausen Castrum in the Roman era?
  
  A. To secure the important transport route along Pfäffikersee
  B. To establish a Roman settlement in the region of Oberwinterthur
  C. To serve as a major trade hub between the Roman Empire and the Rhine region
  D. To protect the vicus Centum Prata from barbarian invasions
  Answer: 