Raiders will invade from the wasteland and break through your Vault Door. They continue moving from room to room, stealing resources and possibly killing your unarmed dwellers. The best way of defense against them is to equip Weapons to your dwellers. Upgrading your Vault door will delay them slightly and give you time to prepare. You can also place up to two guards in the Vault door to act as the first line of defense. Dwellers who'll perform as guards should have the strongest weapon in your arsenal and the best outfit to give them an edge against these invaders. When raiders are invading, your resource meters will slowly deplete including your CAPs so defeat them as quickly as you can.