I took some screenshots while redesigning one of the hub worlds this week. Thought I’d show you a bit of my level design process, and the evolution of the level through some images:
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First, I start out by placing simple primitives to block out the level and get a sense of the space:
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Once I’m happy with the overall layout, I start to replace some of the placeholders with actual assets. Here you can see the pipe getting replaced with actual glass pipe segments (I’ve also added the coloring filter on the player’s camera): 
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During this time, I’m also replacing some of the large blocks with more interesting architecture:
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You can see now that the entire pipe has been replaced:
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However, after playing around in the space for a little while, I decide that I’m not too happy with the design of the pipe. It doesn’t indicate direction. I try something new. The pattern on the pipe indicates direction. Looks a bit awkward though… : 
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Here’s another design that looks a little better
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How does it look indoors?
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Now I start designing the interiors, so rooms don’t all look like giant boxes: 
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Finally finished, here’s a few shots of everything put together:
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Of course, this isn’t by any means the final form of the level. I’m going to need to playtest this quite a bit, and I imagine it will go through at least 10 – 15 iterations before I’m happy with it.