Before launching an attack against enemy Capital city, make sure that there is no small icon of a shield near the city, which would mean that there is the "Ancients’ Protection" in the city.
The "Ancients’ Protection" is a strong army whose vocation is to assist game beginners. It’s better not to attack such cities.

Every player can receive Protection of the Ancients as reward for several quests.
In order to complete the very first quest with the Ancients’ Protection reward you will need to open the City Gate. Consider that Ancients’ Army will arrive in 3 hours after the reward has been accepted.

The Ancients will stay at your Assistant Slots for 60 days.
If a player don’t login the game for more than 8 days, the Ancients will leave sooner.