Wow! that's pretty impressive! Especially the Texture Mixer. I noticed a couple of things:
1) When creating a Wall you can't set the strength precisely, clicking the + and - moves the slider in steps of 10, perhaps that should be steps of 1?
2) When creating a floor, the preview is a bit small for me to see (I'm on a 1280x1024 monitor). Perhaps clicking on the preview window could bring up a larger preview?
3)
Create Floor: I've changed the floor texture path in settings but it's still opening up the default path: 'Textures/Floor Textures'.
4)
Create Sign: I've changed the default texture path for 'Sign Textures' and when I go to create a sign, the big white texture box is empty and clicking on it brings up this error:
Runtime Error 506 - Could not load image at line 717
You've done a realy good job here, well done!
Search for "HellHouse" in Showcase.