Other ways to join cardboard together can include plastic zip ties, which can be good for making flexible joints, so if you want to make a wearable building or shelter zip ties are a good option. The horse head below has been constructed from cardboard sheets and then all joined together using zip ties.
Sticky tape is a quick and easy way to join cardboard together both for small pieces and for whole boxes.
Taking cardboard boxes apart is a good way to see how they are held together, sometimes boxes are folded and tabs used to keep their shape and you can look at many other things to see how they join together and they might just work for cardboard as well.
If you decide you want to do further work on your shelters and buildings or get inspired to make more complex structures there are many options one being to visit your local Maker Crate where you can get access to a laser cutter and start designing and fabricating your shelters. If you search online you can find heaps of information and helpful advice as well as some pre-designed plans for cardboard buildings and construction pieces.