Mettaton’s Tile Puzzle:

0:00 What is the tile puzzle?
2:11 Inspiration
4:15 The rules (by Papyrus)
5:42 Simplifying the rules
7:47 Writing the maze generator
11:29 Writing the editor, level codes
13:25 Share codes
14:06 Writing the game, movement
15:21 Improving the maze generator
17:50 It is done.