If you were looking for a medieval battlefield game for iPhone, you won't need to wait anymore. "Knight Withstand" is probably what you were looking for.
 you will have role of a knight who is attempting to overthrow the evil king that is oppressing the population into poverty and misery. To end the king’s reign of terror and free the population you need to conquer and take over his castle. 

"Knight Withstand" is available  for free in iTunes:

https://itunes.apple.com/us/app/knight-withstand/id1013160537?mt=8

For more info about the game, we suggest you read this article from developers' website:

http://trafalgarsoft.com/knight-withstand-a-unique-and-fast-paced-arcade-that-will-keep-you-hooked-for-hours/