Citadel's are the key to the game. Control the citadels, and you'll control the universe. Let your opponent control them, and you'll be lucky to survive at all, much less prosper. Here is some info on what the different citadel levels do, and how to develop them. Written by Stephen Whitis.

A. Citadel Descriptions.

A planet with no citadel has no defensive capability. You can leave fighters in the sector, but fighters on the planet will not defend and, in fact, can be taken by anyone who lands on the planet. Since there is no citadel, you can't use any of the citadel commands, such as the treasury or remaining overnight.
 

Level 1 citadel doesn't add to defense at all - it is still defenseless. You can use the treasury, remain overnight, etc. But remember, its not very safe!
 
Level 2 citadel has the beginnings of your defense system. Fighters on the planet will now defend. Usually, you will want to leave your military reaction set to 0%, because the fighters will get 3-1 odds. Occasionally you may want to use some of the fighters offensively, meaning they will attack anyone trying to land. They get 2-1 odds, but it may give your opponent a surprise!
 
Level 3 citadel includes a Q-cannon. You can set it to fire at anyone entering the sector, or anyone trying to land, or both. Firing at ships in the sector uses a lot of fuel ore for the damage it does. Firing at ships trying to land or invade is much more effective, as they are closer when it fires. Q-cannons can be bypassed by a photon missile, so don't rely on them alone. But they can help your defenses significantly.
 
Level 4 citadel has a planetary transwarp drive. With this Twarp drive, you can move your planet to any sector where you have dropped a fighter, as long as you have the fuel. It uses a lot of fuel to Twarp a planet, so use this sparingly.
 
Level 5 citadel means that you have installed a PSS - Planetary Shielding System. You move shields from your ship to the planet (10 ship shields will create one planet shield) and the PSS uses those shields to protect the planet. As long as you have a L5 citadel, all planetary shields will have to be destroyed before anyone can invade - and each shield will take 20 points of damage to destroy.

Level 6 citadel equips the planet with an Interdictor Generator. If turned on, this generator will make it difficult for your enemy to escape from your Quasar Cannon. The IG uses fuel ore to power it.

B. Citadel Interest

In 1.0x the citadels earn 4% daily interest. In 2.0 citadels earn a meager 2% daily interest regardless of the planet type :(