My idea:
To unlock this mode, you first need to kill the pears in learning mode in order. Then, when you go to the Carrotian Council, there will be an open block in the top-left corner where the Season Key will appear. This will open a door somewhere in the early caverns that has 4 blue circles, each with a symbol above it.
Summer (sun symbol):
As you play the game, it gets darker and darker as the noonday sun sets over the horizon. The longer you take, the darker it gets. If you play long enough (2.5 hours?), it will begin to brighten again as the sun rises.
Autumn (leaf symbol):
The wind blows, so you, enemies, and items slowly drift to one side if you stand still. The wind direction changes on each level.
Winter (snow symbol):
Everything is covered in ice, making it slippery and hard to control yourself.

Spring (flower symbol):
Flowers sprout up along the platforms, making it sometimes hard to see where the enemies are behind the foilage.
After you beat all the levels, at the very end will be 4 new keys depending on the season: The Sun Key for Summer, the Leaf Key for Autumn, the Cold Key for Winter, and the Flower Key for Spring.
Thanks for this fantastic game with all it's side-routes that are never boring!
P.S. Who makes hammerfest? Is it still updating?
Greatings Hihahammer
