I wanted a version of the interior castle wall without the alcove. I svck in Blender and such, so my solution was to snag a flat wall off of an existing mesh and paste the branch into a copy of the alcove one. A little stretching and placement, and - BAM - instant flat wall. The collision, though, is still set up for the alcove, so in-game you can step through that section of flat wall and into the alcove.

Just using NIFSkope, what would be the best way to give the new wall branch collision so that in-game it acts like a wall?

Sorry for all the questions lately - I'm getting into more advanced stuff lately than I usually do.

You're right, that is pretty advanced stuff and not easily explained in a post.

There's a http://tesalliance.org/forums/index.php?/tutorials/article/37-blender2oblivion-part-2/ that does contain what you're looking for. Scroll down a bit till you come to the section titled Creating Collision for the Cube Stool. It explains in detail how to create collision from scratch in Nifskope. Ignore the fact it's for a cube stool, the principles are the same. Remember to delete the existing collision of your mesh first amd make sure Draw Havok is on.

Good luck.

I would definitely suggest that you modify collision in Blender instead of doing it in Nifskope which is way more complicated. If you need help I can guide you through step-by-step, but you'll have to give me some screenshots because don't have the files here.
Basically, if I understood what you want, it's just a simple case of selecting couple of vertices and moving them. (just one of the ways to do it)

Easiest method using NIfskope would be to find another piece that uses a collision box ( this is easiest since all the settings will already exist ) or add a collision box shape to the Nif) and then adjust the placement\size settings of the box ( just need to adjust the XYZ location of the 8 pixels making up the box so that they align with the wall properly. If you can upload a copy of the mesh for download we could add the piece and then give you directions (easier to explain using actual settings for a specific mesh so node #'s etc. can be included in the directions ) - so that you could then do the same.

If you need some help with it just upload the adjusted mesh to a site like 4shared or similar and post a link here.