how to unlock this is sparta achievement

Seems like there is an invisible wall for the enemys when pushed out of the wall preventing from conquering this is sparta achievement
About the 'This is Sparta!' achievement, I tested it in different castles and cities but it is not working, it's as if when kicking the enemy he hits an invisible wall. I got the achievement by luck trying to kick them from that wooden ladder structure (not the simple ladder they climb but the one that is surrounded by wood)
I tried to kick them off this wooden ladder structure at first, but when I kicked, my character started going down the ladder, so I went up again and attacking, and my character went up and down until one of the soldiers fell and the achievement was unlocked