Hot glue is the way to go for stuff like that, hands down. The cheap sticks have no give, so any weight shift and they just snap clean off. For something like a castle tower, I would have also added some tape on the inside seams to take the strain off the glue joints. Cardboard can warp and pull apart the adhesive if it's too dry or too humid, too. Sounds like you did the smart thing by testing it on a real build, even if it was the hard way. Next time do a quick drop test on a small piece before you commit to the whole thing. That little bit of checking saves a whole lot of embarrassment on stage.