First, I love the game.  It is a lot of fun.  I find it really annoying how few a number of defenses you can setup around your planets, particularly early on however.  Anyone can walk through.  Coming to your home system at least should require more of a force than it presently does.  Setting up a good defense shouldn't have such artificial limitations.  Please change the system to something more like the ship mantenance cost.  Maybe a defensive cost/penalty for the number you build over a certain number?  It seems crazy that you get so few.