To Occupy your first Expansion Dungeon you will need to complete enough of the Quests in order to reach level 10. You do not gain Experience over time until you reach level 10. The only way to get Experience before you are level 10 is to do the Quests. Once you reach level 10 then your Dungeons will produce Experience over time, based on the Rooms you have built. Completing Quests after level 10 will award additional experience rewards to advance faster. Players earn Dungeons through Leveling. To view when you can occupy additional Dungeons, review the Leveling Rewards.
There are many opinions on the Resources Dungeons should contain. Which Dungeon you take is up to you, but is greatly discussed and disputed among players.
First Expansion Dungeon: The most basic Dungeon you take should include both Iron and Crystal, at minimum, or you'll spend a lot of time Shipping Resources into the Dungeon to make Room Upgrades. A preferred Dungeon will also include Primordial Earth or Primordial Water, so that you can Upgrade your Rooms even further, as well as make Steel Ingots and Crystite. Any Dungeon that includes Iron, Crystal, Primordial Earth or Primordial Water and any additional Resources besides Gold, which is guaranteed in every Dungeon, would be a superior second Dungeon. Do not concern yourself with taking Dungeons with high tiered Resources, or you could harm your progression.
After occupying your second Dungeon, you can start the Surveying for a third Dungeon, for when you hit level 20. It takes most Overlords approximately two weeks time before achieving Level 20. Once you've Surveyed a Dungeon, the results will be recorded in the Info tab, under Surveys. This will allow you to quickly search for a specific Resource Node from the Dungeons you have already Surveyed. This tool will be very valuable as you pick your 3rd, 4th, 5th and so on. This will allow you to recall Dungeons that may not help you on your 2nd or 3rd Dungeon, but you want to remember for a later occupation.
Second Expansion Dungeon It is also recommended for your third Dungeon that you also pick one that has Iron and Crystal in it, along with other Resources, in order to avoid excess Shipping of Resources. Iron and Crystal will become hard to keep in supply, if you don't have Mines in each of your first three Dungeons, as you use a lot of it to build your troops and Rooms. For your later Dungeons, you can start picking ones without Iron and Crystal. You will be Raiding more often, at these levels, and building up Iron and Crystal through Raiding. If you Raid inactive Dungeons, you can gather a significant amount of Iron and Crystal very quickly, as most inactive Starter Dungeons usually have these Resources accumulating. Raiding Settlements also builds up Iron and Crystal pretty fast, as well. By this level you will have better Research skills that increase Mining rates, higher level Mines, better Furniture to increase Mining rate, and additional Goblins, so the rate at which you produce Iron and Crystal from the same Dungeon will increase significantly.
Once you reach level 20 you might want to start looking towards specializing your Dungeons for when you hit level 30, so that you can get the most out of your Dungeon Tiles. A level 5 room takes up 25 tiles, so you can't have a level 5 room of every type in each Dungeon.