As far as I can tell, once you have a non-aggression pact, and certainly once you have an alliance, with an NPC you can move thorugh his territory.  This is pretty important since otherwise finding all the enemies is next to impossible, at least on large maps with many opponents.

However, there are parts of the map where the terrain narrows. This can be an isthmus, a mountain pass, or a combination. 

How do you move past an allied city that blocks such a  choke point?

You can not teleport (that can only target destinations you control.)

You can not "lower land" to remove the mountain (that apparently only works within your territory as well.)

And you can not move into the city.

Is there a way around this?

Thank you,

Joel