There Is A Table With A Cake On It On Level 2 (Main Street)
- Clear Up To And Encompass The Cake Within Your Walls And Light The Candles.
Build A Siege Breaker/Robot Combo And Drive The Robot Over To The Table.
- Make Sure The Robots On The Opposite Side Of The Table From The Giant Present
- I Do Not Know If This Is Required, But I Did It.
Make The Siege Breaker Stay Put, Then Go Recruit Some New Kids From Outside The Walls.
- When One Of The New Idle Villagers Walks Past The Lit Birthday Cake, You Should Be Able To Pay For It Again (3 Coins)
Unlocking The Mount (And Maybe Paying To Ride It? So - Give Or Take 4 Coins) Nets You This Achievement.
Good Luck!