Properly defending your dungeon may be a priority if you are in an active region. The main objective in defense is getting your units to be aware of the attack as quickly as possible. By placing rooms in which your units frequent between the entrances and such rooms as the Vault and Storage, you can increase the chances your combat capable units see the intruders before they make off with your precious resources.
There are also several crafted items that can assist you. Alarms, traps, and totems can be strategically placed to help insure your victory against your attackers.