I love love love this idea. The colours are great, they work really well. The most obvious go would have been alarming red tones but instead you chose cool blue that gives a feeling of inevitability, some calmness to the piece.
The empty space above and below give nice scale to the image and put emphasis on the scene. The laser beam from the skyscraper is a nice detail.
But I have to explain why I gave just 2.5 stars for technique. You have neglected depth of field. The tentacles and the city are in the same pane yet the tentacles are very sharp and the city is blurry, the amount of sharpness/blurriness should be the same. Changing this would also change the role of the model in the piece.
If she'd be sharper than the rest of the scene it would put the viewer in her shoes.
If she'd be the one out of focus then the emphasis would be on the city and it's futile attempts at defending itself and we'd have to guess what role the woman plays in this - has she saved herself or is she the beast's master?
Another suggestion would be that the moon should be higher up to balance out the image, the deep black below.
But over all this is a very nice piece, I liked it once I laid my eyes on it - the idea here is great enough that the technique seems unimportant. And yet it pays to get all the details.
Sorry if I'm very messy about this critique, this is my first one so please bear with me. :3

I really loved the image.

The colors is too good, reinforce the idea of "apocalypse" or something like that...
Looking closely I see some minor issues on the space ship but as I say it don't really matter so much...

On top left corner, there some cloud that look a little desaturated, and on the below, the city and the others elements looks too dark, I think a little bit more of light, but only a little...

The tentacles are awesome, no words about it.
My final words are: I would make some changes and some more details without importance, but the beginning is there.

Nice work