Thanks for the advice. But in this case, I didn't have time to do anything at all.
After three turns, a red Knight appeared on the horizon, not very strong, but I was completely defenseless, so I ran towards my main castle and hired a weekly reinforcement in the captured town. In the end, I didn't capture the red castle, but after a turn, a blue Knight with a crazy army arrived (he already had 3 Archangels and 78 Archers, and it was only the 3rd week!). He captured the town and almost attacked my main castle. So now I've been holed up in my territory for two weeks, leveling up on neutral creatures. I have no idea what to do next.