Have a look at this incredible article. Very interesting I must say. Here's a part:
"First, let me point out something that Ender's Game got right and something it got wrong. What it got right is the essentially three-dimensional nature of space combat, and how that would be fundamentally different from land, sea, and air combat. In principle, yes, your enemy could come at you from any direction at all. In practice, though, the Buggers are going to do no such thing. At least, not until someone invents an FTL drive, and we can actually pop our battle fleets into existence anywhere near our enemies. The marauding space fleets are going to be governed by orbit dynamics – not just of their own ships in orbit around planets and suns, but those planets' orbits. For the same reason that we have Space Shuttle launch delays, we'll be able to tell exactly what trajectories our enemies could take between planets: the launch window. At any given point in time, there are only so many routes from here to Mars that will leave our imperialist forces enough fuel and energy to put down the colonists' revolt. So, it would actually make sense to build space defense platforms in certain orbits, to point high-power radar-reflection surveillance satellites at certain empty reaches of space, or even to mine parts of the void. It also means that strategy is not as hopeless when we finally get to the Bugger homeworld: the enemy ships will be concentrated into certain orbits, leaving some avenues of attack guarded and some open. (Of course, once our ships maneuver towards those unguarded orbits, they will be easily observed – and potentially countered.)"