I think the engineers should ignore the walls. It's an attack-move order, which means attack first (reclaim for engineers) and move if that's not possible. You can see this with other units like medium tanks or artillery - if an enemy is in range, they attack. If not, they move. So it seems to me that engineers should move around the walls if there's a route available, since they can't attack it.
I'm a noob though so I don't know if engineers try to reclaim enemy structures when they're in range and on an attack-move order. If so, then the engineers should probably try to reclaim the walls since they're just another enemy building.