Had a little help from @Pamar and Kid Redd, and some very, very vague spots of distant memories. Total time is 6h 9m, but that does include a few short breaks and re-playing an end game save to get the other possible endings as well.
Going to install Ascent now as well, but doubt I’m going to be any near as efficient with it.
This bit took ages to figure out, and it was only by pure chance that we did.
You need to go up in the central lift. Then, before getting out the lift at the top, press the horizontal bar lift button, then get out before the doors close. You’ll hear a little warning siren. The lift decends to an in between point and if you turn around again to face it you should be able to walk onto the top of the lift where you’ll find the controls to rotate the fortress.