I am finding this quite annoying..... I am playing as Rome and have taken large swathes of North Africa. Carthage keeps sending armies deep into my territory, sometimes three regions in. In game terms this is hundreds of miles bypassing lots of cities along the way. I am missing the old AGEOD concept of forifications "blocking" these raids. Is there any such restriction on this behaviour? At present there is no such thing as the extremeties of the Roman Empire beating off all invaders, they just march in when they please and strike at the regions way behind.
I find decent garrisons have an effect, say 5 mediums and 10 skirmishers, that tends to either slow these incursions or stop them (depends on how/if the battles resolve). Its also one reason to build up your fortifications
On the other hand - they are often then out of supply and you can pull the same trick. I've sometimes assualted and taken 2-3 regions in a turn using this.
But isn't that exactly what Hannibal did? Bypassing fortified cities etc. As long as they don't take those cities you as the Roman player should be able to catch these forces (Roman's usually have higher movement ratings, also you should have interior lines and a good road network). Whether those deep raids are a good idea for the ai is another question. If weak forces they won't do much damage, if strong they will starve to death...