I want to hit a city with my Assassins BUT I get this message .....

'The owner of this city is currenly protected from Assassin attack. If their protection has not expired  by the time your attack arrives, any greek assiaasins sent will be lost with no damage dealt to the enemy. Are you sure you want to continue'

I assumed the shield / bubble was in place - BUT - when I checked there is no shield.....

I repeated the attack attept and checked many times over a 15 minute period - still the same - no bubble - how come? What has the defender set up??