Morten St George Theory

[Linda]

 Except for Montséqur, the Cathars did not build castles, but when regions converted to their religion, they acquired the older castles that were there. Montségur was rebuilt and expanded by Cathars in the early 13th century, then destroyed in 

That seems fair enough.

Here is a real medieval depiction of the Cathar Montségur: 



Do you see a coned tower with balcony and window in your depiction? But I see one in the VMS depiction.

I see two towers and no swallowtail merlons. So all you can be going by is the one tower, if that is the only depiction you have. 

Given the shape of the current chateau, i doubt anything shaped like the vms fortification would have fit in its place.
Also, none of that explains all the raised walls that seem to extend from it in various directions. You said they didnt need them, due to the steepness of the location, so why are they in the vms?