This was not a dirty trick, but it's equally cheap.

It was in a desert province, I don't know which one anymore. My garrison consisted of one unit of feudal spears and one depleted unit of archers. I was attacked by a Egyptian prince (with some stars) and his retinue. I don't know anymore how such a small battle came about but it could've been a close fight because of the extra valour on the prince.
I stationed the spears on a good dune with the archers behind them. But there was a sandstorm and I couldn't spot the prince. As I was the defender it would be foolish to seek him out. I just sat there until the timer ran out. I never saw the royal knights.