Leveling up base defenses will increase their attack power, health points, and their attack range.
All enemy ships MUST defeat all Orbital Defenses before they can start attacking your Space Station. Therefore, even having a few high level Orbital Defenses can provide precious time for your allies to retaliate and defend you.
However, when attacking an equal or lower level player, defensive buildings are usually destroyed in 1 or 2 attacks, so many players choose not to invest much of their money or time into leveling them.
The Meteor Star is a defensive structure that prevents fleets from travelling in a particular direction. Although the path of the fleet will not change, it will have to encounter the Meteor Star before proceeding to its target. The Meteor Star is an excellent way to protect your Defensive Structures as your opposing fleets will have to destroy your Meteor Stars. It can allow 1-2 more rounds of firepower untouched from your Defensive Structures if placed properly. Meteor Stars are easy to destroy, but as the Meteor Star is leveled up, it becomes more of a BLOCK in the way for opposing fleets while your cannons and AA Guns do their magic.
The Particle Cannon is a great attack weapon for protecting your planet. Although it has the weakest attack of all of the defenses, it does not have a cooldown period. This weapon does have the highest range available. Against more sophisticated ships, this weapon has trouble penetrating shields, but when the shields are down, this can be a great attack weapon.
The Anti-Aircraft Gun is a great attack weapon for protecting your planet. It has the most powerful attack second to the Thor Cannon. It also has a cooldown period of one round. This weapon will attack one square in all directions at point of impact within its maximum Attack Range, thus it can hit multiple ships at the same time.
The Thor Cannon is the ultimate attack weapon for protecting your planet. It has the most powerful attack and radius of attack, but it also has the longest cooldown period of two rounds. This weapon will attack any and all enemy ships within its Attack Range at the same time.