Outpost Defense:Outposts have no permanent defences. If there are no ships stationed in the system, the enemy may engage the outpost or blockade it. Engaging it would let them destroy the outpost and all the buildings and population. Blockading it would require the fleet to have a special cargo freighter. Each turn the outpost is blockaded, the resources that it produces are stockpiled on the enemy cargo freighter instead of being transmitted to the main planet. This includes minerals, research points, energy, and food (if possible). Blockades are effectively raiding the outpost for resources, which can then be sent back to another planet and acquired. Minerals and food are absorbed into their reserves, RP is added to their galactic total and energy I'm not sure what to do with.
Colony Defense:The main colony in each solar system can build a starbase, ground-based defences and planetary shields. If friendly ships or a starbase are present, the enemy have to first destroy it. If no shield is present, the planet can then be bombed. If a shield is present, the enemy fleet can select to siege it. Sieges are an automated process in which the enemy each fire 100 volleys per turn and the shield defends against it. Planetary shields will have a massive amount of hitpoints, a small hitpoint regeneration amount per turn and huge energy requirements. Once matter-energy conversion is discovered, the regeneration per turn can be augmented by drawing a certain amount per turn from the mineral reserves. The siege mechanic gives time for any remaining ships to mount a defence.
Starbase: Orbital battle station.
Planetary Shield: Basic planetary shield.
Shield Regenerator: Boosts shield regeneration per turn.
Matter Convertor: Once built, minerals from the material store can be converted into shield HP at a selected rate per turn.
Multi-phased shield matrix: Maximum shield HP increased by 50%.
Ground Batteries: Weapons on the planet that can fire at ships during the siege.
Stellar Convertor: Matter-energy conversion based plasma cannon. Costs 50 material from the planet's reserves per shot.
Defense Satelite: Convert a moon into an orbital weapons platform. Number that can be built depends on how many moons a planet has. Massive energy requirements.