This is a work in progress. I will add the remaining heads as soon as I find out how to unlock them.

If you can't unlock them due to various reasons (i.e. missing the Steam sale, don't own the other games, etc.), you can always purchase them at the Steam Community Market or trade with other people using one of the many trading websites (a good starting point would be the Trading section).

I have put up a link to the Prisoners' names so that you can purchase them directly on Steam Community Market. No need to navigate further!

These heads can only be obtained by buying games made by The Behemoth.

• Knight
  
  
  
  
  
  
  
  
  
  
  • Unlocked by owning the game "Castle Crashers" in Steam.
    
  • Cannot be bought / sold over the Steam Community Market (although there are some that can still be bought).

These heads can be unlocked by playing the game, mostly Furbottom's Features, or simply launching the game during specific time periods.

• 50% Off
  
  
  
  
  
  
  
  
  
  • Unlocked by playing the game during the Steam Summer Sale 2014.
    
  • Cannot be obtained anymore ingame.
  
  
• Alien
  
  
  
  
  
  
  
  
  
  
  • Happy Anniversary! Unlocked by playing the game during August 7, 2014.
    
  • Cannot be obtained anymore ingame.
  
  
• Barbarian
  
  
  
  
  
  
  
  
  
  • Unlocked by finishing one level of Furbottom's Features during August 22, 2014.[forums.thebehemoth.com]
  
  
• Behemoth Chicken
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during May 5, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Blocketeer
  
  
  
  
  
  
  
  
  
  • Unlocked by finishing the Co-op Challenge or a Furbottom's Feature stage during July 28, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Boom!
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during July 4, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Boot
  
  
  
  
  
  
  
  
  
  • Unlocked by playing Arena during June 6, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Buckle Your Pants
  
  
  
  
  
  
  
  
  
  • Unlocked by finishing the Final Stage (aka Time Attack stage) of Furbottom's Features.
    
  • Can also be unlocked by finishing one level in Furbottom's Features.
  
  
• Donut
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during July 19, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Duck Shark
  
  
  
  
  
  
  
  
  
  • Unlocked by playing the game for the first time.
    
  • Can also be unlocked by playing Furbottom's Features during a specific time period.
    
  • Cannot be obtained anymore ingame.
  
  
• FBI Agent 1
  
  
  
  
  
  
  
  
  
  • Happy Anniversary! Unlocked by playing during August 7, 2014.
    
  • Cannot be obtained anymore ingame.
  
  
• Furbottom
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during August 29, 2014.
  
  
• Gerard
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during May 30, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Giraffey
  
  
  
  
  
  
  
  
  
  • Happy birthday Castle Crashers! Unlocked by launching the game during August 27, 2014.
  
  
• Jonny Four
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during August 15, 2014.
    
  • Cannot be obtained anymore ingame.
  
  
• I SEE YOU
  
  
  
  
  
  
  
  
  
  • Unlocked by finishing the Final Stage (aka Time Attack stage) of Furbottom's Features[forums.thebehemoth.com].
  
  
• Little Feets
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during August 22, 2014.[forums.thebehemoth.com]

• Manbirth
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during June 13, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Meebs
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during June 20, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Moose
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during July 28, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Mr. Peabody
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during August 5, 2014.[blog.thebehemoth.com]
    
  • Cannot be obtained anymore in game.
  
  
• Paul
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during May 30, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Peeps
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during Jun 20, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Sausage
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during July 4, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Snail
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during August 1, 2014.
    
  • Cannot be obtained anymore ingame.
  
  
• Star / Pluto
  
  
  
  
  
  
  
  
  
  • Unlocked by playing the game for the first time.
    
  • Cannot be bought / sold over the Community Market.
  
  
• Snakeface
  
  
  
  
  
  
  
  
  
  • Unlocked by finishing the Final Stage (aka Time Attack stage) of Furbottom's Features this week (July 11 - ???)[forums.thebehemoth.com].
    
  • Can also be unlocked by finishing one level in Furbottom's Features.
  
  
• The Dad
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during June 13, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Tiny Cyclops
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during May 23, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Triple Threat
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during July 11, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Toothbrush
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during August 8, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.
  
  
• Troll
  
  
  
  
  
  
  
  
  
  • Unlocked by launching the game during July 19, 2014[forums.thebehemoth.com].
    
  • Cannot be obtained anymore ingame.

These heads can only be unlocked during special events (e.g. Steam Summer Sale 2014).

• Hatzilla
  
  
  
  
  
  
  
  
  
  • Unlocked by crafting Summer Adventure Cards during Steam Summer Sale 2014 and getting the Rare BattleBlock Theater item.
    
  • Since the promotion has ended, the only way you can get one is by buying it through Steam Community Market or Trading.
  
  
• Lava Lamp
  
  
  
  
  
  
  
  
  
  • Unlocked by crafting Summer Adventure Cards during Steam Summer Sale 2014 and getting the Extraordinary BattleBlock Theater item.
    
  • Since the promotion has ended, the only way you can get one is by buying it through Steam Community Market or Trading.
  
  
• Scuba Steve
  
  
  
  
  
  
  
  
  
  • Unlocked by crafting Summer Adventure Cards during Steam Summer Sale 2014 and getting the Common BattleBlock Theater item.
    
  • Since the promotion has ended, the only way you can get one is by buying it through Steam Community Market or Trading.