Defend your Indipendant People City

Add the possibility to defend your indipendent people city (client state only) from siege by other empires, and add grievances (for stopping the attack and pay some money for example) agains the attackes as soon as they attack, not waiting that the indipendant people have been captured for that (only for client states again).
Maby also constuct a military base or something like that in the city to control the defence if they are attacked ?