XofDallas wrote:To my mind, there has to be some mechanism, probably located in or under the Temple, by which the orbital defense system can be turned off.
In all probability, that's exactly where it's located- just can't quite see the Go4, or the temple guard, for that matter - allowing folks to go wandering around looking for the off switch.The idea of a second command center hidden somewhere in the Mountains of Light makes sense, but IIRC there's no textev to suggest one.
While not necessarily 'invulnerable' per se, it's not going to be taken out externally with the means and methods currently available to Merlin or the Empire.