A highly enjoyable platformer.
The world: The gameworld is made up of a 'grid', and you transition from each section to the next. There are two main zones, the overworld and the underground. The overworld is very diverse and changes very often, but, in my opinion, provides the best environments. The underground consists of several different environments and are where many of the secret passages are found.
The music: Very simple and short segments are played every so often, but on the whole the music adds alot of atmosphere both to each area and the entire game. Also, the ambient sound is very good.
The inhabitants: In the gameworld, various creatures can be found both in the underground and overworld. These include forest-dwelling peoples to strange horse-like animals with hats, and everything between. They add much of the atmosphere to the game, and discovering each animal is a treat. As a side note, I just wanted to say this game led me to buy Shadow of the Colossus, a game which I enjoyed very much and cherish.
So, give Knytt a try, it's an excellent game.