Here is the full list of all 20 Knytt Underground achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Find the hidden ending
Ring every bell in Chapter 3 without paying a guard
Collect a hidden symbol
Find a hidden recipe
Complete the Figure Space
It has music by D Fast
Complete the Mini Figure
It's just sitting there for some reason
LAZA KNITEZ!!
The proper way to say "I'm sorry!"
Find music hidden in the sky
Complete a hidden challenge in the sky
Strange things happens around him
Find the hidden Juni story in the credits
Find the hidden sheep