Why was roman empire hard to defend?

The factors that made the Roman Empire hard to defend were mostly based on communication and transportation. The empire sprawled from the Atlantic Ocean to the Black sea with the city of Rome roughly in the middle. If a communication were to go out from Italy to say, Egypt or Syria, it could take as long as two weeks to get there, depending on the weather, of course. The same amount of time for a return letter. The time frame was slightly better in Europe as the messages went via the roads. However no matter how good the Roman roads were, the horses could only go so fast and so far in one day. The transporting of extra troops to a trouble area was also slow and cumbersome, depending on the number of troops moved and the distance they had to travel.