A player’s rank can be found out by pointing at the player’s domain on the Kingdom Map or in the Ratings Window.

Players can attack Ruins or Cities of players with ranks (levels) higher than theirs.

You cannot attack Ruins and Cities of players with the ranks (levels) lower than yours.

There is a REVENGE option available in the game. If a lower rank player has attacked your city, you receive a one-time possibility to get revenge. You can attack any city of the attacker, during 24 hours since his attack.

NOTE! Rank Rule doesn’t cover Outer Domains and Camps. Any player can attack any rank Mine, Pit, Salt Lake or Camp.

After a player reaches certain rank, the spell of compassion is cast upon the player's Capital. The required rank is Suzerain.
Ranks System.

Stars Rank From, Rank Points
Esquire 0
Captain 60 000 000
Master 125 000 000
Marshal 350 000 000
Suzerain 650 000 000

Rank_Points = Main_Raiting + (Army_Strength_Rating * Coefficient)

The coefficient is 80.
You can inquire into how many Rating Points each unit adds here.

Information on how many Rating Points remain to the next rank you can find in the Rating Window.

Attack against Cities and Ruins:
Conformity of ranks is checked only once – at the time of launching an attack. If a rank changes after mission have been launched and sent, the attack will happen all the same. Attacker’s rank change will not actually affect the battle result.
Your troops that are to join initiative mission will join the mission even if rank changed.

Ruins' Attack: It can happen so that Ruin attacks player of not suitable rank.
Monsters choose victim and check rank only in the moment of attack planning.
After the planning and until the moment of launching the attack some time passes, during which the player’s rank can change.