I've started playing this game just this week.
It's pretty good overall, but the defence sections are a huge pain.

The only thing I have to put down is the turrets, from the build menu.
Their engagement range is less then half the area I can see while camera is pointing straight down!

They SHOULD be shooting stuff at least as far away as I can see in that straight-down camera position.
that way I might actually be able to compete one or two of these defence missions.