Why can enemy units phase right through my walls in LOTR: Battle for Middle Earth II?

I haven’t played this one in quite a while, but I recently came back to it. Fun times. anyway, occasionally i’ll have an enemy (I play with the A.I.s seeing how next to no-one plays online anymore) be able to move right through my walls that I have built as if they weren’t even there.
It really messes over my strategies.
Is this a glitch, or is there something I’m not understanding?