I have recently painted my collection of Heretic Astartes which contains both very old models -- like the Space Crusade dreadnoughts -- and some very recent ones. I feel their design went the whole circle and returned to its roots. Excellent!
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. »
To make small, round shapes out of cardboard use an office hole punch (available also with funny hole shapes nowadays) or better still a DIY punching tool used to make holes in leather belts and the like »