Just finished my Scottish campaign (btw, it wasn't that hard as I had feared, had problems mostly initially, when I was playing tug-of-war with the Picts).

Once I was besieged in a fortress by the Irish. Since it was some low level (a fortified village, IIRC) it could house only 3 out of my 4 units. So when these three were done with, my fourth unit (the archers) got to the field in the way of reinforcements - from beyond the edge of the battlefield. I marched it to the village - and was defeated totally.

But I now I realize that I could have hidden it somewhere in the woods and given more headache to those dratted Irish. Perhaps it was the way the AI uses when it is in similar situation. Then a unit comes from behind the horizon and hides somewhere and you have a problem with finding it.

I hope I shed some light on the problem discussed.