Idk why, but I see the Planetary Fortress as being an Artifact creature instead of a land. And just an idea, but maybe make it an Artifact Land, like Darksteel Citadel.
True, but some should be Artifact creatures (Planetary Fortress and Missile Turret), and others artifact lands (the Supply Depots). Also, As maybe an additional effect, make the Planetary Fortress require a Sacrifice of a Command Center, as they require one to be built.