Slitherine

I've just been raided by what appears to be 18CP neighbor army, even though I stationed a 16CP army to defend the region (with no defense buildings) against raids. How is this possible? What determines a successful raid? Can you defend with army against raids? How much CP does your army need to have to never be raided again? Does it matter if your army is garrisoned or not?

Did you set your army to "Retaliate"?
That's one way to teach the raiders a lesson and cause some losses to them. Be sure, however, to turn off "Retaliate" if you plan to march into a province whose owner you are at war with because it will cause your army to return to where it came from.
Size and type of units in your army give different deterrent values, don't remember them off the top of my head though I believe light cavalry is pretty good.
There are some military building types that have value as deterrents (Watchtower etc...)
You can also try getting relations to 30+ as they'll stop raiding you.

IIRC (haven't checked it for a while) the formula is something like:
(Total of residing Army Skirmish value) + (Region Defenses *3) + (Special Buildings), which is then compared to the total of the Raiding Army offensive value (not skirmish value; in this case you defend with the skirmish value, but Raid with army offensive value), plus or minus a random factor.

In essence, build those walls, even a palisade makes a huge difference. An army without skirmishers (especially Light Horses are good) is of little use (unless they are Roman Legions, who have good skirmish values too compared to other Heavy Infantry) to bolster defense vs Raids. And if you can get up that Watch Tower or Guard Tower, do so. You can consider yourself lucky, if in a newly settled region you first to get to build a Palisade (or you are Rome and can build it at will) and then get to build the zero-slot Guard Tower -> the region can't be raided by small forces anymore.

The most effective way to prevent raids is to set up a buffer of Client State regions. That way, none of your regions are subject to raiding.

This thread was super helpful.

Swuul wrote: ↑Wed Dec 29, 2021 11:36 am IIRC (haven't checked it for a while) the formula is something like:
(Total of residing Army Skirmish value) + (Region Defenses *3) + (Special Buildings), which is then compared to the total of the Raiding Army offensive value (not skirmish value; in this case you defend with the skirmish value, but Raid with army offensive value), plus or minus a random factor.

Where can I find this hidden information? It's not in the manual.