Originally posted by Luthien:

Oh okay, yeah I put vertical/horizontal beams in every window frame, that doesn't seem to help. Is it a regular wood vs core wood issue? You're open shelters seem to have core wood so maybe I need to construct my top tier with that instead?

In your picture at the very bottom I see what looks like half walls without a horizontal beam across the top of them. (same with the bottom of your top half walls.) Without a framing along its exposed top/bottom, half wall=no wall for shelter purposes.