Walkable.dds is a file which contains, as the name kindly suggests, the area which is walkable on a map. Here are simple instructions on how to edit that file, in order to add or remove walkable areas to our new maps and thus removing some pains in the ass such as Brangulvs Dam and Athos Refuge.
1. Download a program called
DXTBmp2. Install it and and start as usual

3. Select File - Open

4. Find the map folder you want and select walkable.dds (you might need to change file extensions to get it visible). Also make sure you have unpacked this file out of the prelude.pkg

5. Find the 3rd icon at the bottom (Send to Editor), this should open up the image in MSPaint

6. Zoom in on the area you want and little by little add white dots to places you want to be walkable
7. Save the image in Paint (just regular save will do)

8. Click the 4th icon at the bottom in DXTBmp (Refresh after Edit).
9. Save the walkable.dds via File - Save in a
DDS 888-8 32 bit format. Replace existing while when prompted.
10. I rule