I wanted the player to be able to make small notes in each cell of possible numbers, which meant that there had to be 9 spaces in each cell. Having decided that each cell would be a block of 3x3 tiles, I discovered that only two thirds of the puzzle would be visible on the small screen of the GBA at any given time. Thus, the mini-map was born. I decided to add a toggle-able HUD which contained a timer and the mini-map, either of which could be turned off from the pause menu.
The game was controlled by moving a cursor around the screen. If you moved the cursor to an edge of the screen then the view would scroll, while moving the cursor over the timer or mini-map would cause them to jump to the other side of the screen so that parts of the puzzle weren't covered up.
Having made that on-screen keyboard, I wanted to get it into the game somehow. So I decided that each of the 9 puzzles would have a best time. If you successfully completed a puzzle faster than the current best, you'd be congratulated and asked to enter your name for the new high score. But the best times weren't saved to a file, meaning they reset each time the game ran.
You can download a zip file containing a rom of the game and a PDF instruction sheet here. You'll need an emulator - I recommend Visual Boy Advance.