Yesterday we updated such a thing as AI. Earlier in the game, defense in enemy cities was static. Now we have reworked the system and added artificial intelligence (finally! :))
Now the enemy will build armies in the territories under their control and regularly attack you. You will be able to see where the potential strike will be directed and prepare for it by building defense in the necessary cities.

Don't forget about the Defense Buildings, because now their role has grown. Also now there is a real opportunity not to build defense in all cities, but only in front-line areas, where the enemy is preparing to strike.
Thanks for your support! :)