The Citadel of Knope has historically been very poorly understood. It was commonly thought that it would redirect ships to itself whenever your defense force was unable to, meaning it was the last line of defense.
Until recently, it's operation would be to draw down the ship with tractor beams and capture it, thus giving the defender the ship that had just attacked. However in doing so, the building would typically be damaged, thus reducing it by one level, or even destroying it totally.
Scientists now belive that they have discovered the true operation of the Citadel and that they may be able to operate it far more efficiently, and without damage.
Rather than drawing down the ships, it actually repels them, causing damage to the ships engines due to the heterodyne of the repulsor field and the ships engines. Such ships then have to return at one quarter speed back to their spaceport to have their engines retuned. This effect has not proven to be successful on Piloted ships such as the Sweeper or the Fighter. It has however proven to be effective in keeping ships like the Snark and Seekers from hitting their targets.
The only effect this has on the Citadel of Knope is that it requires a cool-down, the period depending upon the level of the building. A level 30 will cool-down within 30 seconds and then be ready to repel another 120 ships, a level 1 may take up to 15 minutes before it is ready again.
We will let you know when scientists are ready to release their findings so that you can safely upgrade your Citadel.