On Hard, the towers cost more than normal, the bloons move faster, and you have 150 lives. Winning the Hard difficulty awards you a gold medal for that track
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~ Description in BTD5 when mouse scrolls over the Hard Difficulty icon
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If you're playing on Hard, you've done well to get this far, but it will all end next level.
Hard Difficulty is the third and final difficulty in the Bloons Tower Defense 2-5(not BTD5 IOS) since the release of Bloons TD 2. At the beginning of a game, the player has half as many lives on hard as they do on easy. In BTD2 and BTD3, hard difficulty has 50 Rounds. However also purchasing the towers is pretty expensive to buy in hard difficulty.
In Bloons TD 5, the player must pass Round 85, which is the first round with a ZOMG bloon, in order to win on hard difficulty. The Bloons move a litte bit faster than on Easy Mode or Medium mode. Towers are 8% more expensive than in Medium Difficulty. This difficulty is represented by a Robo Monkey, or by a Sun God in Extreme Tracks in its icon.
Tips
Due to the fact the you have much less lives than other modes, Spike Factories should be placed at the end of a track to ensure that no bloons pass the end the of track.
Upgrades or Towers that slow down bloons, such as Glue Gunners, are recommended due to bloons moving faster.
Banana Farms are a must-have here, due to the large amount of money needed for powerful upgrades.