It depends on you the whole space station, which is preparing to attack the aliens. Until the enemy launched an attack, it is worth to spend wisely on defense. The most vulnerable part of the station - it's core, it will try to hit his opponents. Most rational way to protect is to install defensive structures at both ends of the core. For the destruction of enemy ships, you will receive the money that can be spent on new defensive turrets, repair and modernization. The game has tremendous momentum and excellent execution.