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To open the area editor, simply double click on an area in the left hand list, or Right Click → View Area on it. You can in NWN:EE open more than one area if you enable it in the options (and can then copy things between them even) but this does tend to cause more crashes so be careful. However it is very useful.
The area editor is no longer a like-for-like visual representation of what the level looks in game due to improvements in lighting, shaders and other things. It is worth looking at the Test Module page for guidance on how to test how areas look and feel.
The camera usually acts as if it is anchored to 0.0 Z height which can be a bit frustrating for people used to a more advanced modern engine or 3d modeller program like Unreal Engine or Blender and sadly does not supported WASD or arrow key movement.
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