Futuristic castle is an oxymoron but as it looks cool it is something you may want to build. This is how. The guide may be useful for any walls, not just fortresses.
1. This project expands upon the imperial bunkers/bastions. You will need four of them for a standard setup. You may build them at the same time as the walls.
2. Download plans in PDF. Note that they are scaled to be printed on an A3-sized paper sheet and that they are for a single wall section (so you need to have four for a classic castle shape).
3. Cut appropriate pieces from a cardboard sheet.
4. Glue the pieces together:
5. Add some details as a gate. I made a portcullis from toothpicks. It does not make sense (would have to slide sideways) but it does not matter as the entire castle does not make sense in the age of rocket backpacks, antigrav tanks, teleporters and city-levelling guns ☺
6. Finish the walls with one of the methods for simulating concrete.
7. Have fun!