Thanks for clearing that up, guys! I followed the advice and added the tile set to the final art piece.
The required challenge is a tile set.
The optional bonus is to show the tiles mocked up.
The team didn't really discuss the possibility of doing just a tile-based mockup.
I think where we have landed is that we won't reject mockups that clearly show the tiles...
...but if you can include the tile set as a tile set somewhere (e.g., adjacent to mockup, or as the preview image) that's better!
Does the final artwork need to show the tiles separately layed out on a grid (as a proper tile set) or can it be simply a scene with the tiles clearly in use?
oh wow this seems like a fun challenge! I never designed tiles for myself before!
Any restrictions on the perspective (top down, isometric, etc) ?
Sorry for the confusion, glad it all worked out!