From GOTM18 description
Because this game is on Monarch difficulty, you have a one hundred percent (100%) attack bonus against barbarian units. This bonus does nothing to benefit you on defense when the barbs attack your cities or your units. Compared to last month's Gotm17-Carthage game that was on Regent difficult you have about a fifteen percent (15%) lower chance of easily killing the barbs. Remember that this advantage only applies when you attack barbarians.
Forgive me if this is common knowledge, but from a quick test I just did, the bonus against barbs applies when you are on defense as well.
Just sharing the info, because I have been taking the above quote for granted, and it has been affecting my strategy against barbs, probably getting some of my units killed for no reason.
