Complex sentence: A full-size fort, or "castrum", was started late in the 2nd century: this date is derived in part from a re-construction of a uniquely detailed plaque, fragments of which were found by archaeologists in the 1960s.
Split into simpler sentences: A full-size fort, or "castrum", was started late in the 2nd century. This date is derived in part from a re-construction of a uniquely detailed plaque, fragments of which were found by archaeologists in the 1960s.