A chink in the armor: Another Roman blunder

During the early stages of the Roman empire, Rome was  building up and tallying many enemies as it expanded. The number one threat from military conquest in my opinion was the Carthaginian empire. Although the empire was directly across the Mediterranean, the great general Hannibal, decided to take a different route, the north. He decided to attack Rome by crossing the Alps in the north and head directly to Rome, essentially leaving Rome on the brink of being conquered. Luckily for the Romans, Hannibal did not have any siege equipment. Fast forward a bit into the later stages and decline of the Roman empire. The barbarians to the north began to push down to avoid the threat of the Mongols coming and spreading into their lands. The barbarians used this chink in the armor to wear down Rome over time, which eventually contributed to its decline. To me, I am not entirely sure why Rome did not put more emphasis into a reinforcing of the norther borders while they were strong and at their peak. Knowing that Hannibal was at the walls before with the northern route, why did Rome not take that into consideration when preparing defense at all? Perhaps they tried but failed. Nonetheless a big chink in the armor indeed.