How do I unlock all the secret characters?

How do I unlock all the secret characters in You Must Build A Boat?

Solution 1:

• Catapult - Unlocked in Woodward's library after unlocking enough monsters.
• Ninja - Hides in the scenery next to your boat in the Pagoda map.
• Wraith - Kill a wraith in the Vault map with a Fireball spell.
• Black Dragon - Watch the paintings in the Gallery map, and tap the one that's a painting of a Black Dragon.
• Geltower - Eat food while fighting a Geltower inside the Mage map (the big fluffy cube)
• Robber - Tap your character repeatedly, making him hop at least 5 times just after you hop out of bed.
• Skeleton King - Sell enough items to Saundi's Broking, at least 50+. I'm guessing it's "every type of item" but I haven't been able to corroborate that.