A kingdom can use any extra power they are producing towards powering the defense energy shields. You have the option of powering Military Shields, Missile Shields, and/or Probe Shields.

Probe Shields:

Probes communicate with each other using an encrypted frequency transmission. By flooding space with random transmission blasts we can disorient the enemies probes and cause the failure rate to increase 20%.
Maximum Effectiveness: 20%

Military Shields:

By rebounding the enemies offensive battle carriers your kingdom's defenses can decrease the offense value of the enemies forces.
Maximum Effectiveness: 10%

Missile Shields:

These shields are specially designed to detonate missiles upon contact, before they make it to your kingdom. At full power, these shields can stop up to 100% of all missiles launched at it. Great if a whole sector is attacking you.
Maximum Effectiveness: 100% (except on Server 3: 50%)

See Also:
Missile | Attack | Kingdom Status