Thanks, i'll take a look at that solution, but :
Can you have 3DGS display inside some Windows Forms ?
Or it is only calling 3DGS functions ?

The SDK is from 2010, and not official , it don't contain samples... not easy, that's why i bet lot more for the editor on another engine using C# directly like Axiom or the abandonned TrueVision 3D.

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That's some option i'm looking at.
if it is too much complicated i will use one of these options :
- only cubes and not using corners, the easy way !
- Use simple predefined rounded cubes ?
- Use detailled complex predefined models : corridor pieces, room pieces

Even TorchEd (TorchLight level editor) is not so easy, when you want to make mor ecomplicated stuff :
TorchED tutorial