Cut it out of a single sheet of A4 photocopier card.
The castle folds flat for posting - just push the towers towards the centre, behind the drawbridge.
Here is your freebie design:
If you are digi-cutting the castle out, print out the .pdf of the design first and use it as a folding guide. You will need a steel ruler and a fine-point embossing tool to mark and crease the folds.
Finishing touch: the pennants in a contrast colour. Roll them around cocktail sticks and tape behind the towers.
If you would like to learn how to design your own one-page pop-ups,
papercraft guru Paul Jackson is your go-to guy. Here's a pic (and link) to his indispensable book, Cut and Fold Techniques for Pop-Up Designs: