Step 4: Glue on little reinforcing strips
This seemed to add a lot of extra strength to the structure without being too time consuming or difficult. Part of this I did while gluing the main pieces together.
Note the strips of cardboard in picture 2 and 3. I folded these and glued them at the points of the open triangles where the windows and door were.
Next, I cut random strips of cardboard, folded them, and glued them over seams that seemed a little weak. This seemed easiest if I glued on one side of the strip to one panel, held it until the glue set, then bent the strip back to apply glue to the other side and pressed it to the adjoining panel. It can be tricky to try to hold a bent strip to two panels at an angle without one becoming unglued or something. See pictures 7 and 8 in this strip for what I mean about gluing one side at a time.
I also glued a bottom to the door side to stabilize the structure. I was too lazy to try to glue a bottom to the whole thing; I didn't have a huge single sheet of cardboard and it seemed like more work than it was worth. You can see the bottom in picture 1 of this step. Note the triangular corners from the bottom piece that I folded up and glued to the outside of the structure. One side was a bit small so I glued another strip of cardboard over it to reinforce that one.