What is the 17th achievement in Bloons TD4 for iPhone?

Question

I have played through all the levels and challenges on the iPhone 4 version of Bloons TD4. I have only received 16 of 17 achievements, and the last achievement is not visible. Game center shows 1000 of 1000 points earned, and googling didn't turn up any answers, so I'll pass it to you, what is the missing achievement from the list below? Is it a confirmed typo of some sort?

I have recieved

• Learning the Ropes
• Experimental Upgrade
• Ocean Breeze
• Ranking up
• Top Rank
• Monkeying Around
• Advancing on the ...
• Expert
• War Veteran
• Bloon Buster
• Exterminator
• Moab Mauler
• Elite Agent
• Sun Worshipper
• Shock and Awe
• Branching Out

Answer 1

The secret 17th achievement is Temple Monkey:

Temple Monkey: Unlock the secret of the Temple Monkey!

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The secret is... (mouse over to see - you may want to try to get it yourself before reading any further)

Get 40 towers and 30,000 bananas on the Temple Monkey level (the last level).

It will ask if you want to upgrade the Temple Monkey; click yes. Doing this will sacrifice the first 40 towers you built, including banana farms.

The Temple Monkey shoots two attacks which look like the plasma super-monkey's, except that its attack pops anything in one hit, including MFBs or dirty bloons

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BEST WAY TO GET THIS ACHIEVEMENT

You don't have to beat the level, just get the Temple Monkey, so the easiest way is to start out with a few (four) overpowered boomerang-monkeys, and get an upgraded banana farm. Then just to spam cheap dart monkeys and banana farms until you have enough towers and can afford the Temple Monkey.

This is almost trivial if you have the exploding dart upgrade, as it makes the non-upgraded dart monkey the best tower in the game for its price. You can literally beat an entire level on hard with nothing but exploding-dart monkeys and banana farms.