On Hard, the towers cost more than normal, the bloons move faster, and you have 100 lives. Winning the Hard difficulty awards you a gold medal for that track
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~ Description in BTD5 when mouse hovers 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 Bloons Tower Defense 2-5 (not BTD5 Mobile) 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, with subsequent entries increasing the required rounds to be beaten before winning. Towers and their upgrades also face an increase in cost, as well as bloons having their speed increased.
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 little bit faster than on Easy Mode or Medium mode, and the player has only 100 lives. 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.
Bloons TD 6
In Bloons TD 6, the player must pass Round 80, which is the first round with a ZOMG bloon, in order to win. The player also starts on Round 3, meaning the player gets 2 rounds less of cash, starting with only $650. Towers are 8% more expensive, and the player has only 100 lives. There are also a lot of variations based on Hard difficulty.
Alternate Bloon Rounds: All rounds are slightly harder as some bloons are replaced with tougher variants.
Half Cash: All income sources are halved. Players start with $325 cash instead of $650, or with $425 instead of $850 if the necessary Knowledge is added. Must complete Double HP MOABs to play.
Impoppable: One life, survive until Round 100, very expensive towers. Similar to BTD5 Mobile's Impoppable mode, but you start at Round 6 instead of 3. Must complete Alternate Bloons Rounds to play.
Double HP MOABs: All MOAB-class bloons have double health. Must complete Magic Monkeys Only to play.
C.H.I.M.P.S.:(No Continues, Hearts lost, Income, Monkey knowledge, Powers, Selling). Extremely similar, but not identical to NAPSFRILLS. The hardest mode in the game, survive until Round 100 with these rules. You start at Round 6.
Sandbox: A mode to test out strageties in different tracks, difficulties, bloon rounds, and towers. No rewards whatsoever in this variation.
Tips
Due to the fact the you have many fewer lives than in other modes, Spike Factories should be placed at the end of a track to ensure that no bloons pass the end of the track.
Up amount of money needed for powerful upgrades.
See Also
Impoppable: The hardest difficulty in BTD5 Mobile and a fearsome challenge in BTD6.
Before Version 2.0, Half Cash was "Half Starting Cash", where only the Starting Cash was reduced. This was changed ever since Version 2.0. In addition, Half Cash is now requires both Magic Monkeys Only and Double HP MOABs to be completed.
Half Starting Cash used to be before Double HP MOABs though.