The 1.4.0 update to Royal Revolt 2 in September 2014 added the dungeon to the game. The dungeon is a major single player campaign that is accessed by clicking the gate in the center of your castle guarded by two paladins.
You can unlock the Dungeon once you reach level 4.
The dungeon contains numerous chambers for you to attack. Unlike normal raids, these chambers are not created by other RR2 players. Instead they are pre-made raids with predefined rewards. Other than being pre-defined they function much like normal raids. You will be placed on a path and must proceed along it to defeat a castle. Like a normal raid you will receive a fixed gold bonus for completing the attack, although the gold rewards seem to be much lower than for attacking other players. Also, the first time you complete an attack on a chamber with three crowns you will receive a bonus reward. This usually takes the form of extra gold, gems or an item. Occasionally you will be rewarded with access to a new monster unit type or an upgrade to an existing monster unit type.
Each chamber in the dungeon needs to be accessed. This requires using a worker to dig a tunnel to each new chamber. There is no gold cost associated with digging, but a worker is occupied for a period of time creating the tunnel. You are not informed in advance how long the digging will take, so check the table below or be prepared for some unpredictability with your worker allocation. You can, as usual, use gems to speed up the digging.
To execute an attack, each chamber has food cost that must be paid each time you invade. Once you have successfully achieved a three crown completion the gold reward for any chamber becomes very small relative to its food cost; if you're looking for gold, you would be better off attacking any opponent. Even a very badly failed raid will generate more gold revenue than a second attack on a chamber. Still there could be a reason for such attack - sometimes it has big medal reward and may help you to win high place in a league tournament.
You can enter the Cave of Grave Goods even if your inventory is complete full as you get a slot for the new item for free. You can only get the free slot if your inventory is full when you enter the chamber.
Here is a list of the various dungeon chambers and the associated rewards. Sorting the table by "bread" gives you the order in which you can enter the chambers - whereas some chambers are sidetracks to the shortest path to all monsters. Under Chamber required you can see which chamber you need to finish with 3 crowns to unlock that chamber. There are 90 chambers.
The normal gold reward for each cave is based on your Hero's level. It differs for each player.