Barbarians are a constant threat!  Be sure to guard yourself against them well, as they can even enter your suburb from neighbouring suburbs!  

 

Increasing your Fort and Legion blocking radius is a good way of defending your buildings. Once a Barbarian comes within range of your blocking radius they will be forced to do battle against your Fort. By making sure you have strong Fort defences, you give yourself the best chance of defending against these attacks.

 

Increase your Fort points available and the size of your blocking radius through research.