Good on you for having the courage to go first up! Much respect
I like the geometric basis of the design. Imperial stuff does tend to be a blocky, and heavily geometrically based.
Empire jiggy 1 Empire jiggy 2 Empire jiggy 3 Empire jiggy 4 Empire jiggy 5 Empire jiggy 6 Empire jiggy 7 Empire jiggy 8 Empire jiggy 9
I think, however, that while basing your design strongly on geometrics is good, you need to be careful to make it not look like it's made out of duplo (not saying yours does, it's just a potential pitfall).
Also, be careful about putting down some blocks without thinking about what they are there for. You don't want the weaponry and radar that you'll put on to feel like an afterthought. I can see that this is an early concept, so I understand why you have ommitted those details.
I am interested as to what the big horizontal cylinder is for? Is it like a tunnel or entrance? Is half of it meant to be obscured under the ground, so it's actually a half cylinder? And the sphere, what is that for?
I like the sphere that closes up. It reminds me of a building from TA, the moho metal maker:
YouTube - Arm Moho Metal Maker (recorded it for you

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The way that works definitely opens up some doors for you with the vertical limitations of your building, don't you think?
Before I move on, I need to ask you, and anyone else entering here, permission to host their images on imageshack or elsewhere, so that I can post them into the other competition forums.
People from outside this forum cannot see the images without registering here first. Let me know ASAP!
(or even better, host them somewhere I can link to here, so I can just nick the links!

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Dr.Zais down at Moddb has put something out too:
http://img174.imageshack.us/img174/6514/base1hl2.jpg http://img251.imageshack.us/img251/297/base2aw2.jpg
Very interesting landing idea; good level of completion and detail. I think the greater form needs a little bit of work, though.