The rack, iron maiden, prison door, and other props were leftovers from Irwin Allen's sister series, "Time Tunnel."
Robot sings, "Oh, you take the low road and I'll take the high road and I'll be in Scotland before ye." That garbles the song and loses the whole point (which might be deliberate). "You take the high road and I'll take the low road, and I'll be in Scotland afore ye". It is after a battle far from home. The high road means the road of the living, the road of our step-by-step world. The low road is the path of those already dead, on which the dead can travel at high speed.
Per the title, astral travel refers to times of out-of-body experiences perceived as unfolding in environments other than the physical world, by an astral counterpart of the physical body that separates from it and travels to one or more astral plane.
This is an episode Guy Williams (Professor John Robinson) does not appear in. His voice is heard in one scene, but he does not physically appear in the episode.