Collect as many cardboard boxes as possible!
I end up getting the majority of mine off Facebook marketplace and my local Harvey Norman store also gave me some of the large fridge boxes.

Supplies

Lots of boxes! And an industrial tape dispenser.

Step 1: Assemble Boxes

The boxes were all stored flat packed, so the night before commencing the project assembled all the boxes and stored them under the patio.

Step 2: Commence Construction of Outer Walls

Decide how large you want to go with the calculated amount of boxes you have. Then start with the outside walls. I have allowed for a door where I will be building a drawbridge a little bit later on in the project.

Step 3: Continue Building Up

Continue building the outside walls till you teach a desired height.

Step 4: Securing As You Build

I'm using an industrial tape dispenser with over a dozen roles of tape in supply. And also used a few tent pegs to secure the bottom layer to the lawn.

Step 5: Internal Rooms

With all the external walls complete it's time to use up the remaining boxes and build internal rooms to your own desire.

Step 6: Slide Through Tunnel

I taped two fridge boxes on top of each other, to make a tower and then cut a door either side of the bottom so that as you go down the slide you slide through a 'tunnel'. I also added another smaller box at the end to make the tunnel a little longer.

Step 7: Functioning Drawbridge

Almost finished the project, with the final step of building a functioning drawbridge. I taped several pieces of cardboard together so that they were thicker and more structurally sound. And also put some reinforcing on the edges. Then used tent pegs to secure the bottom to create a hinge.

Step 8: Adding Rope

Securely tape some cardboard rolls to underside of the entrance to allow the rope to slide through and then secure the rope at the end of the drawbridge.