Tactics for the Stronghold.

I recently noticed a peculiar thing about playing as Gurnisson. Let's say he has birds and a ballista in his army. We attack monsters (cargi-harpies). The birds wait, the harpies fly closer, and then we fly the birds to a corner of the map. The cargi-harpies stand next to the ballista – and, lo and behold, they can't attack it. This makes it very easy to shoot down harpies (I haven't noticed this with other monsters) with the ballista.