Aside from completing the main story levels, there are few extra challenges and hidden items that can be accomplished to unlock special content. On each level, there is one hidden item called a Tuku, which when all 6 have been found, unlocks the ability to play any level with all of Rau's weapons.The challenges are known as Baumusu's Challenges and each stage has unique set of challenges that can be viewed during the game from the start menu. The challenges ask you to either perform a number and type of stealth kills, use Kuzo to find some icon or object, or perform a certain number and type of combat combos on enemies. If you successfully complete the challenge list for a level, you unlock that level in the arena. The arena can be accessed from the tavern in the game by talking to the old sage up stairs.
In short, everything. This guide will walk you through all six levels, showing you strategy's and locations for good stealth kills. On every level, the Baumusu Challenges will be completed and the voice over describes the requirements and gives tips on the best places to beat these challenges. Additionally, there is an Arena Challenges guide that shows you how to defeat both Time Trial and Body Count modes while completing the Arena Challenges. Yes, it can be done - some of the later level arena challenges are quite tough!
In summary, this guide shows you: