Your ability to defend your Galactic base depends on how well you build structures that defend it. Click on turrets to build turrets around your base. To build defenses, you need to have enough crystals and energy for it. If you don't, build Power Stations. This will raise your energy rate and let you build more turrets. Once you have reached this point in the game, you can build refineries to mine crystals. When you have more crystals, you can build better defenses. Build a wide range of turrets to be able to take on attacking land and air units.