At first I ran the game in VisualBoyAdvance-M and noticed some drawing on tiles outside of the viewable area.
![](https://i.imgur.com/ySsoLsG.png)
As i didn't want to dive into the memory map, I opened the rom in bgb's vram viewer, as this can show all tiles at once.
![](https://i.imgur.com/7nACYbQ.png)
Finally, I printed out the tiles and did some puzzling :D
![](https://i.imgur.com/IOA0eKo.jpg)