This was designed to be an easy first project for people to learn to design PCBs in KiCAD. It only uses very standard parts, all in the KiCAD standard library; LEDs, resistors, and 2.54mm headers (all THT). The PCBs are assembled to be board tiles, X pieces, and O pieces. The pieces have LEDs and an X or O (sort of) pattern, which light up when plugged into a board tile.
After building the tic-tac-toe game, I realized I could make more arbitrary structures with the same PCB, by assembling it in different ways. I then made a slightly modified design to make it slightly more versatile for structures, and with the tic-tac-toe markings removed.
The PCB has a grid of 3x3 LEDs in the middle, a single resistor for the LEDs, 3-wide headers (GND-VCC-GND, symmetrical, so they can be plugged in either way) in the middle on each side for board tiles to connect, and a set of vertical headers in each corner to plug the piece tiles into.