Ooh, nice 

 .  Definitely the full sphere flare over the half-sphere.
And those minis are very good.  Do you want to tweak/re-render them or are they already "premium" rendered?
The BTG walls look the right shape, but I'm thinking they need to look a bit less wispy.  How about adding a green mesh inside the grey stuff, for a hint of the organics that go into BTG production.