You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Rhine

Context:
From the death of Augustus in AD 14 until after AD 70, Rome accepted as her Germanic frontier the water-boundary of the Rhine and upper Danube. Beyond these rivers she held only the fertile plain of Frankfurt, opposite the Roman border fortress of Moguntiacum (Mainz), the southernmost slopes of the Black Forest and a few scattered bridge-heads. The northern section of this frontier, where the Rhine is deep and broad, remained the Roman boundary until the empire fell. The southern part was different. The upper Rhine and upper Danube are easily crossed. The frontier which they form is inconveniently long, enclosing an acute-angled wedge of foreign territory between the modern Baden and Württemberg. The Germanic populations of these lands seem in Roman times to have been scanty, and Roman subjects from the modern Alsace-Lorraine had drifted across the river eastwards.

Question:
When did the Rhine stop being the Roman boundary?

the empire fell