In Astro Empires, defences are special Structures These structures help to protect your bases from enemies trying to conquer them. They serve the same function as normal structures. Each level requires area, population and energy. The higher level defence structures only require Energy.
When a defence structure is built, it effectively builds five "units" of the same type. These act like individual units for all intents and purposes. For example, on the right of the structure view on the planet overview:
The Missile Turrets are level one, yet are displayed as 5/5, increasing each level by five. The next would be 10/10, the next after that 15/15, and so on. This system is used as a means to mitigate any difficulties in effective pricing formulas. For example, an individual Missile Turret lacks the ability to destroy anything more than a few bombers, while a group of five can easily outmatch such a force.
For more detail on using defences please see the Defence Guide.