Happened to me when sieging Constintanople as the Huns. Next thing I know I got attacked from the sea, and land by supporting armies (so 3 armies attacking me overall). My troops ended up being on water (just like the enemy army). I could move them to the ground during deployment, but not during the battle itself. Also, enemy ships got stuck on an invisible waterfall...
EDIT: The entire problem is recorded on my polish twitch:http://www.twitch.tv/kommissarzbignievov/b/625946294
- 35th minute mark