I'm chasing my enemy two valleys away (over 24 hours to get there) to wipe his army out in his new remote city. Luckily, one of my allies whose city is right next to my enemy's remote city can receive my army in his city enabling me to have some rest before the final assault.
Sadly, when I select my enemy's city to send an attack there, my army in my ally's city doesn't appear on the choice of armies to send (doesn't appear on the right part of the screen)...

I figured maybe I should then create a tower and try to send my army from there as it will be listed as one of my cities. Once this done, it says "cannot send attacks from this city" or something alike.

I'm now heading back to my nearest ally's city.

How can I attack my enemy while my army is in my enemy's neighborhood?

Any help/advice appreciated...