[Gandalf Parker]

If no unit is there then the first to walk in claims it just as if they were not allies. But if a unit of your allies is there then you cannot combat them.

However....
I have hidden inside my allies fortress. I have stationed troops inside my allies fortress to help defend it when he was weak. And (possible exploit warning) I have done dangerous summonings inside my allies fortress in order to force his troops to help defend me if the control fails.