years ago when I didn't know any better I used Elmer's white glue. I have a Christmas only layout so I put the building away each year. They held together nicely.
Now for the lucky benefit of using that white glue: Eventually I wanted to paint them and do other things to improve the look knowing that I would never want to sell them so no harm. I soaked each structure in water and after a short time they easily came apart - the glue washed off easily too.
- walt