It looks like battering rams can't attack walls: they attack units behind walls. That means if there's only one unit defending, there's only one tile you can siege.

Note that I actually manually clicked the 'attack button' to see if it made a difference.

I think this is a huge design flaw, since if you have overwhelming numbers, you'd want to take advantage of the fact that the defenders can't defend every angle. Right now, one unit can defend a city as long as it can kill battering rams in one hit, since they have to line up one-by-one to take a whack at a wall. 

The fact that melee units can sally from any wall and return behind the wall the same turn with no issues is another issue entirely.

agreed, I think all siege units should be able to attack empty tiles just to tear down the walls

This is still an issue on release, it seems