What are the Robber Baron Castles

Robber Baron Castles (usually called RBCs) are a NPC (Non Player Castles) in "the Great Empire" kingdom (on other kingdoms there are equivalents). Those are computer generated targets which does not belong to any player and appear randomly on the map. They exist in the game to let players attack targets that are not related to other players, to be able to gain coins, resources (food, wood, stone & specific kingdom resource), equipment items, and from time to time even to get some small amount of rubies or time skippers (!) Every RBC (or equivalent version of it in other kingdoms) will "remember" (keep) its current level anytime you logged in to the game.

RBC equivalents in kingdoms

In other kingdoms, there are NPC equivalents for Robber Baron Castles with different names, start and end levels, and of course they give the resources for the respective kingdom.

Attacking Robber Baron Castles

RBC levels are private (unique) to each player (i.e. you only see your own version on them, not other players version). On every attack on them they will upgrade (increase their level). every level has "sub levels" (the number of victories to go the next level). Usually it is referred as 16-2 which means level 16 2 victories to level 17. The higher the level of RBC, you will need more (or better) troops & tools to defeat them. However when they level up they will give you more resources you can loot, better equipment pieces (and sometimes more rubies or time skippers).

After every attack, the Robber Baron Castles (and other NPC as well) have a "cool down" time until you can attack them again (usually is it 3  hours). This time is private to you only.

Using Spy Reports when attacking RBC

For every level (and sub level) of any RBC (or its equivalent on kingdoms) the defense formation (meaning: the troops & tools) is constant and never change for all players (e.g. The great Empire RBC level 16-2 is always the same for all players). This means that you can use "Spy Reports" information (that was generated by others that did spy on them before) on every level/sub-level to attack without spying RBC. This will save you both coins and time. 

Robber Baron Fortress

In every kingdom there is a special "Robber Baron fortress" formation. They have the ability to send small attacks on you, when you attack and defeat RBC on the same kingdom the respective "Robber Baron fortress" will not attack you for for 7 days.

The advantage of attacking those fortresses is that they will give you resources and also a small amount of rubies when you defeat them (!) but be aware that the "cool down" for those is "public" (only in other kingdoms than "The Great Empire") means that is it common to all players, but not "private" like your RBC, so when attacking it might be that another player attack will "hit" before you, and you will get a "no battle" message.