Running through walls

How is it possible for enemy to run through walls ? It has happened couple of times to me now that I built large wall around my contry just to see enemies just walkig through them. Are there any units, strategies or upgrades which allow this ?

It is NOT possible to run through walls and there are no units or upgrades that can do it. There must be another explanation to it. Either the walls wer not tight, the enemies troops were already inside country before walling it or it was a bug.

You can walk through walls. its a bug