The Catacombs beneath your Castle are where you can hide your fighting forces and Resources* in case you are attacked. Units in your Catacombs will be safe from destruction if you are Raided or Besieged - but they cannot participate in the combat.
Click on your Catacombs in your castle. You will see 2 interface tabs:
1. "Info" - Here you can see how many of each type of Resource your Catacombs can protect, based on its current upgrade level.
2. "Units in Catacombs" - This tab shows what Units are currently in your Catacombs. From here you can also send more Units to the Catacombs, or recall the Units so they can fight again.
To send units to the Catacombs, click “Send” from the “Units in Catacombs” menu. To remove Units from your Catacombs so you can use them again, click “Recall” from this menu.
The Catacombs is also where all Units that you buy via Black Market or get as a reward will first appear.
All Units in your Catacombs can also be managed from your Keep under the "Castle" tab. Sending or recalling Units to the Catacombs takes 1 minute. Units in your Catacombs will not be revealed even if an enemy successfully Spies on you.
You can hide any number and type of Units in your Catacombs but the amount of Resources that are automatically stored in your Catacombs is determined by its upgrade level*.
* The Facebook social network uses different raiding mechanics. Catacombs there do NOT protect any Resources and thus, cannot be upgraded. Successful enemy raiding will allow your opponent to take up to 50% of all Resources, stored in your Castle, depending on the raiding party carrying capacity.