|Empire Defense. Strong and Weak Points
With a little bit of chance, you start alone in your sector. You suspect there are several rival empires out there, but they must be in different sectors. We know that the only way they can get to your empire is through Stargates. Thus, by protecting the Stargates located in your sector, you can effectively block out enemy invasions.
To do so, you need to protect the outtermost planet of the system with a Stargate. Building and then upgrading a Planet Cannon on that planet should hold your enemy for a while... eheh!
Outter Planets get attacked first
Planets always orbit around their sun. For an outsider to reach the innermost planets of a system means going by all the outtermost planets first. By placing a Planet Shield or a Planet Cannon on a Planet, not only you protect that planet, but you also protect the inner planets too! The enemy will need to invade the fortified planet before attempting to continue their journey toward the innermost planet.
You know, by looking at the Flight Paths, which stars the enemy will need to travel to first before they can get to your colonies. Protecting the outtermost planet of each of these stars can stop or at least slow down an invasion considerably, allowing you the time to retaliate!