Reach level 10 to unlock Castle Challenges.
Castle Challenges will last for 30 days. A new task will open up each day. If you miss a day you can still complete the missed tasks, but they have to be completed in sequential order.
An avatar is unlocked after every 10th completed task. Once that 10th task is complete tap on the "Get Reward" button to claim the avatar.
To unlock all 3 avatars you must have all of the tasks completed by the end of the 30th day.