Unlockables

Catapult Fill out the library.

Geltower Use food item while fighting it.

Wraith Use a fireball spell while fighting it.

Robber When you are sleeping in the bed in the boat, tap to jump and just keep tapping and jumping before you hit the ground.

Ninja Switch to the Pagoda level. Don't start the level, just switch to it. The Ninja will be hiding in the background behind the boat. Just tap on him.

Dragon He's in the paintings in the Gallery stage. Tap on his portrait.

Skeleton King Have over 20,000 Gold EDIT- I've been told this isn't correct. He unlocked for me after I sold 20,000 Gold in one sitting- with a lot of rare items. Not definitive, but that's what worked for me.