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.
Ring every bell in Chapter 3 without paying a guard
Find the hidden ending
Complete a hidden challenge in the sky
Find music hidden in the sky
LAZA KNITEZ!!
Strange things happens around him
Find the hidden Juni story in the credits
Find the hidden sheep
Complete the Mini Figure
Collect a hidden symbol
It's just sitting there for some reason
Find a hidden recipe
Complete the Figure Space
It has music by D Fast
The proper way to say "I'm sorry!"