Red attacked me only twice... when capturing the second and fourth castles... With that, the sub-mission with Red ended with a checkmark. I encountered strong resistance only twice... Once, a powerful Blue arrived (that's where I had my greatest losses), and the second time while capturing the third (the leftmost) castle.
During the first month, I upgraded and armed only the main hero. And, mostly, I just moved back and forth between two castles. Later, when the strongest enemies were defeated, I started buying an army for a second hero and left him on guard... while I moved further... since logistics were at maximum, no one managed to escape... By that time, I was already level 24, and the enemies I encountered had only a one-week army... In general, the biggest problem was a lack of money :) There are... so many armies in the cities... but they are difficult to buy.. so I didn't buy any until an enemy approached some city...