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.
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, bloons move faster than on Medium, and the player has only 100 lives. There are also a lot of variations based on Hard difficulty.
Half Cash: All income sources are halved. Players start with $325 cash instead of $650, or with $425 instead of $850 if the necessary Monkey Knowledge is added. Must complete Double HP MOABs to play.
Impoppable: One life, survive until Round 100, more 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. Survive until Round 100 with these limitations. You start at Round 6.
Sandbox: A mode to test out strategies in different tracks, difficulties, bloon rounds, and towers. No rewards whatsoever in this variation.
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.