Complex sentence: The fleet was erected in the early 13 BC. The fleet was for the monitoring of the entire Rhine, the mouths to the right of the Rhine Rivers were secured so as to maintain the smooth transit and trade on the Rhine.
Split into simpler sentences: The fleet was assembled in 13 BC. The fleet was for the monitoring of the entire Rhine. The mouths to the right of the Rhine River were secured so as to maintain the smooth transit and trade on the Rhine.