I find that when attacking, the AI always re-deploys to counter the position of my army. Once, as Sicily versus the Serbians in XL, I was able to get on to level ground rather than have to fight uphill. But only after trying to outflank them. I was able to push them, during the battle (which took place in woodland, so their cavalry were screwed ), downhill. Well, to an extent, anyway. But the AI always loves re-deploying to counter my army, always...