How to make that you can't se your allies (no cartography effects)

First, sorry for orthografic and gramatic mistakes, I’m not a native english speaker.

I want to make an scenario with allies, a city that you need to capture, but i want it to be allie so that i can put defensive structures as decoration, but when i use the triggers to make unavaliable carthografy and free carthografy (they are different), or change player visibility I can still see the line of sight of allie. Any sugestion?