Citation: Daniel Konstanski, The Secret Life of LEGO Bricks: The Inside Story of a Design Icon (Unbound, 2022). — p. 88.

Through cutting and gluing existing bricks, designers affixed LEGO® Technic pins and receiving holes to the sides of standard 1x2 bricks. Pins could slide down into holes but still rotate, creating a strong yet mobile hinge. Incorporating such elements into the castle’s side walls allowed them to open, transforming tight interior courtyards into expansive play areas. It was a perfect solution.

The 1x4 Hinge Brick (parts 3830+3831) were invented to allow castles to open up and reveal their interior.