1. Finish the game. (Do not start a new game.)
2. Play the first 5 levels 2 times. (Takes the least amount of time and extremely easy.)
3. A TREASURE BOX will appear above the trader with a green down pointing arrow.
4. Scroll through all previous levels and look for the TREASURE BOX in place of level number. (I don't remember Treasure boxes above the trader while playing but if it does appear go find the treasure box.)
5. Click on the TREASURE BOX and play the level. The exit will have a field star in it.(WARNING NOTE: There is only ONE chance to complete level. No restarts.)
6. Once level is completed a "treasure" field will appear.
7. Collect all the treasure before exiting back to game.
8. Repeat 2-7. (It is a grind.)

Finished game.

TREASURE BOX indicator.

Scroll through previous played levels to find TREASURE BOX

Treasure level has a star field exit

The treasure level

Suggestion: once TREASURE BOX level is completed and the treasure retrieved, click on trader (middle) to see if an artifact can be made.

Artifact screen. Green check marks indicate all treasure needed has been found.