That's the editor for Duke Nukem 3D, which was state of the art at that time. I remember prototyping the levels for my A3 project in Build before finally implementing them into my game. That has been much more efficient, because you could actually see what you were editing without having to guess the correct floor and ceiling heights and textures (because those had to be scripted for each single region before they could be assigned in WED -.-).
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On topic again: Conitec, you should really get your website reworked by some professional agency. And while you're at it, let them also redo the engine logo. I mean, it's really old and... not *that* beautiful.

Just look at your competitor's websites! You could increase your number of customers just by having a more professional internet presence.

I'd even consider buying A8, but as of now, I'm not overly convinced of the product.

the build editor was awesome and way ahead of its time.

it was embedded in the engine, truly wysiwyg and you couldn't simply place stuff (like in unity) but really work on the level geometry.

i think the only thing that is comparable nowadays is the blender game engine but unfortunately it has weaknesses in other areas.

Building level for raycaster engines rocked a lot
I still have my problems working in 3D. Duke3D, Doom and all the like was like dawing a dungeon in 2D and then adding different floor and ceiling heights. It was so much fun. Unfortunately not with WED, which looked like DEU minus most features.
DEU pretty much sucked, I always used Dmapedit for building Doom maps. It had no 3D preview/edit mode, but the 2D part was awesome back in 1994...

ITT: Oldfags talking about the good old time.