Currently, Storm of Wars doesn't support the defender to see an on-going battle in real-time. It's something we will add in the future. 

Note that, when your city is being attacked, you can still enter your city, but can only perform basic actions, for example, moving buildings around or researching technologies. If you put on a city buff, instantly complete a summoning order, or retreat an army home, it won't affect the result of the on-going battle.