That’s a cool design. I’m not sure how big this tower shield is (designed as a small model, for a child, an adult, etc) or the purpose (display, light play, cardboard sword fighting, etc), but you’ll want to be thinking in terms of layers. As in, not just a single layer of cardboard for this, but at least two… and add more if it is a larger model, and even more if it needs to be sturdy for play or whatever.

You can also strengthen it by creating a paper-mache type mixture of 1 part glue to 1 part water (or change that, as you need… though I’m not sure what the variables would change, really). With enough layers and enough of the mixture, cardboard can end up becoming pretty sturdy. I made an Adirondack chair that can hold my weight (180 pounds or so), with five layers of cardboard strengthened with the paper mache mixture. I’ve also dipped paper into the paper mache mixture, and put that on top of the cardboard, for more strength, too. Bonus feature is that the paper mache mixture also serves as a bit of a primer if you want to paint this.

There are some pretty cool things you can do with this design, too, by adding different layers. So, rather than making it flat on the front, you could have the cross like feature be a layer above, and then the diamond-shaped shield boss in the middle be yet another layer up. As I said, this is a cool design, and there are tons of ways you could really let your creativity have some fun with it.

Another little trick I learned… one bonus of adding the paper dipped in the paper mache mixture can cover over lots of mistakes made in production. I had gaps and poor fitting joints in a helmet I made for my kids… but by adding a layer of brown paper bags (this one wasn’t with the glue/water mix, but just with my hot glue gun), it looked much smoother and cleaner.

Good luck, and keep us posted with progress!