I want to know the answer to the following question:
In about 260 AD, what forced the people of Roman towns to find shelter in Roman fortresses?
All the background I'm provided with is that "Repeated raids by the Alamanni tribes provoked the ruin of the Roman towns and economy, forcing the population to find shelter near Roman fortresses, like the Castrum Rauracense near Augusta Raurica.". Is that enough to answer the question?