Complex sentence: Only after the foundation of the German Empire could archaeologists begin to study more precisely the previously rudimentary known route of the "limes" and make first systematic excavations in the second half of the 19th century.
Split into simpler sentences: Only after the foundation of the German Empire could archaeologists begin to study more precisely the route of the "limes", about which there had previously only been a rudimentary knowledge. As a result, they were able to make the first systematic excavations in the second half of the 19th century.