How do I make a nav mesh allow AI to walk on flat surface of a mesh?

Development Asset Creation
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1

I have made a giant castle using some voxel editing software, i have collision working so units stand on it, but they cannot walk as the nav mesh does not allow movement on it. Things I have tried so far:

Set nav mesh to overlap all
Set castle to no collision
set castle to overlap all
Set navmesh to never affect pathing

This is the object im trying to allow pathing on top of:
http://prntscr.com/n3x466

2

I solved my own issue, To do it correctly I had to add a collision box under the navigation menu in the model importer. From there set the box to fit the area you need, make sure the top of the box is where you want things to walk! if you need help with this let me know and I can post some screenshots or directly help you get it working!