Hard Difficulty is the third and final difficulty in Bloons TD 2-5 (except 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. From Bloons TD 4 onwards, more rounds must be completed compared to easier difficulties. Towers and their upgrades also face an increase in cost (by +8% in most games), 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 starts with $650 but also starts on Round 3, meaning the player gets 2 rounds less of cash. Towers are 8% more expensive, bloons move ~15% faster than on Medium, and the player has only 100 lives.
There are also a lot of variations based on Hard Difficulty. Initially, the player must complete Standard Hard in a certain map in order to unlock each variation of Hard on the same map.
Alternate Bloons Rounds: All rounds are changed significantly to make the game more difficult. Bloon types may spawn earlier on the game, and many rounds will feature a greater mix of bloon types with larger or smaller bloon waves.
Half Cash: All income sources are halved. Players start with $325 cash instead of $650, or with $425 instead of $850 if the "More Cash" MK is active. Must complete Double HP MOABs to play.
Impoppable: One life, no lives can be generated, survive until Round 100, 20% more expensive towers than Medium. Similar to BTD5 Mobile's Impoppable mode, but the game starts 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. The game starts at Round 6 instead of 3.
Sandbox: A mode to test out strategies in different tracks, difficulties, bloon rounds, and towers. No rewards whatsoever in this variation.
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.
Impoppable: The hardest difficulty in BTD5 Mobile and a fearsome challenge in BTD6.
Fully unlocked Hard Difficulty menu in BTD6 (pre-3.0 (?))
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.
In Bloons TD 6, a single Ceramic bloon leak on any Hard difficulty game (with default lives and no protection from Monkey Knowledge) is enough to result in a game over. This is because there are only 100 lives on Hard difficulty, whreas a Ceramic bloon has 104 RBE.