Vulnerability Hours are the periods that the structure can be attacked with each slot representing one hour. The amount of hours needed differ between each size of citadels and are managed in the Structure Browser window.
Upon entering a predefined Vulnerability slot the citadel will now be attackable. Citadels cannot be affected by any form of Electronic Warfare and will limit the amount of damage it can take per second as defined by its size. This damage cap cannot be exceeded and will result in decreased damage or zero damage dependent on attacking force.
Once damage has been taken the Citadel will begin a 15 minute repair timer that upon completion will fully repair the Citadel this timer however is paused while taken sufficient damage and should the Citadel reach 0% Shield or Armor (dependent on current stage) it will enter a Reinforcement state.
Reinforcement ends at different times dependent on what stage the siege is at. When Shields are reduced to 0% a 24 Hour reinforcement cycle takes place during which the Citadel will be invulnerable. Should Armor be reduced to 0% a 6 Day Reinforcement occurs (The exception to this however are all Citadels located in a Wormhole. These timers will end after 24 hours)
Once the reinforcement state ends a new repair timer will begin where if successful will fully repair the Citadel and will no longer be attackable until the next vulnerability slot. Additionally a Citadel that has been forced into a reinforcement state will not follow the vulnerability timers until it has been successfully defended and a repair cycle completes.
While vulnerable the current status of the Citadel is shown on the damage indicator.
The outer grey/red circle shows the current damage. It has one section for shield (top), armor (lower left) and hull (lower right). In each section a dot represents 1/12th of full health. A grey dot means intact, a red dot means damaged. So in our example there are only 2 grey dots left on the lower right section, which means the Citadel has only about 17% (2 x 1/12 x 100%) hull left.
The inner yellow circle shows the repair timer. In our example it has about 2/3 of the 15 mins left. One can hover with the mouse over the Citadel damage indicator and it will display how much time is left on the repair timer and whether it is running or paused.