I've been playing around with a few occlusion culling solutions and I think I found one that I like. Occlusion culling is basically an optimization technique that stops the video card from drawing objects you can't see. Without it, the card will draw all of the buildings even when you are inside the storehouse. This is currently how HammerHelm works.
As you can see in the gif, as I move into the storehouse, the game turns off the buildings I can't see! It's also turning off the trees, rocks, and other objects that would normally be drawn in the camera's view.
I'm still experimenting with this and need to make some tweaks to get it to work even better. I also need to test it with the new trees. But I think it looks promising and should provide a nice boost to performance, especially on lower end PCs.