This video by Wicked Goodies shares a pair of tricks for building stable gingerbread houses, techniques called Sealing & Edging.
https://youtu.be/vLZj6GNIxSsSealing
After baking, an easy way to preserve and protect your gingerbread house parts is to ice the back sides of each one. The underside of a baked gingerbread cookie has very porous surface. It’s the weakest part of the cookie. By sealing it off with icing, the gingerbread becomes sturdier and less vulnerable to bending or breaking. I use a small offset spatula to spread royal icing out to the edges on the backside of every piece, then leave them to dry for a couple of days.
I prefer to use chocolate or brown colored royal icing for sealant because it blends in better with the color of gingerbread.
Edging
Once all my parts have been sealed on the back and the sealant has fully dried, I like to edge every piece as well. Edging involves using a sharp serrated knife to bevel the inside edges just enough so that they fit together flush. For instance, two walls that meet at a 90° angle are each beveled at 45° angles so that they meet in a V shape. So if you have four walls, from the top view, they would meet like this. When you edge, you want to shave at the sides very gently with your best serrated knife. I only recommend attempting this method if the parts have been sealed. I also recommend keeping extra gingerbread dough on hand the first few times you try it because you might break a couple of pieces before you get the hang of it.
This technique is what makes tight seams and clean lines in cookie architecture entirely possible.
If you’d like to get my recipes for dough and icing and learn more about how I bake, construct, and decorate gingerbread houses, check out my books. I’ve included their links below.
BOOK #1 LINK:
https://www.wickedgoodies.net/gingerbread-house-dough-recipe/BOOK #2 LINK:
https://www.wickedgoodies.net/gingerbread-house-decorating-building/