City occupied by an ally

I'm asking a question about a particular game situation, without (apparently) an answer on this forum. I was allied with a player who decided to betray me. From this point of view, nothing to say: it happens and I don't have nothing to say about. But the turn when he broke the alliance, he entered in my walled cities (but without people inside) with his troops and he took them. From the next time, when I play allied, I leave almost a unit in cities to prevent this situation. My question is: is it legal or could be subject to any game rules?
Thanks in advance for answers,
pierre

You cannot do anything against this kind of backstabbers, best solution is, as u mentioned, leaving 1 unit in each city. Allycapping is just meant to let you give some of ur lands to ur ally without having war and is useful in team games. Next time make sure to either find genuine ally or leave units in each city

You could always avoid allying anyone. This way everyone is your enemy and you know what to expect. Or you could plan to backstab them first(although you become the very thing you swore to destroy). But unfortunately there's no way to stop someone from backstabbing.

Thanks Same e PleaseMe for the answers! I still have a lot to learn...