Hey, 

This is a weird issue, and the first time I've noticed it, I probably wouldn't have if it weren't under such extreme circumstances.  I've just started a game, and had war declared on me very shortly after entering the ancient era.  The AI attacking me has 2 scouts and 2 warriors.  I only have 1 warrior defending my city, however, I'm on an extreme chokepoint where they can only attack from 1 tile downhill.  It seems like I could actually win this battle, but the AI has resorted to cheating lol.

Basically what happens is, I place my unit in the choke point, and start the battle, the first turn resolves and they attack me uphill into fortifications. I come out on top of that exchange by a mile, then just proceed to defend and hope I can hold them off.  Since it's early the combat only lasts 2 turns max, which should be doable.  If I defend and end my turn the AI just doesn't play while I'm watching the battle.  If I minimize the battle to go do other things and then come back, they have magically teleported a unit up a cliff next to me that they should not be able to reach as my unit is in the choke and completely surrounded by cliffs otherwise.  (I thought maybe they were having a hard time deciding what to do so I reloaded and sat in the battle for 15 minutes while writing this to see if they ever moved while I was watching, no dice)  The only other way up is a long trek around the cliffs, which actually aren't even in the battlefield, so there is no way up there without cheating.  

It's funny how they only do it when I'm not watching though, but this is literally the difference between me holding my only city, and losing it.