For the fortress I was thinking maybe a makeshift city built on a ship floating above Earth
[hider=Like this]
[img]https://s-media-cache-ak0.pinimg.com/564x/f6/f0/c8/f6f0c8088c8eb26c6f1ce5921d49f812.jpg[/img]
[/hider]

Or maybe have the machine people already build an artificial planet. Kinda like Cocoon in Final Fnatasy XIII
[hider=Cocoon]
[img]http://vignette4.wikia.nocookie.net/finalfantasy/images/8/88/Coccon_Exterior.png/revision/latest?cb=20130325094207[/img]
[/hider]

And have said artificial planet have state of the art facilities and futuristic look.
[hider=Like so]
[img]http://www.finalfantasy.net/wp-content/uploads/2011/10/w19.jpg[/img]
[/hider]

[sub]Yes, I'm obviously a final fantasy fan. :newlol[/sub]