History ThatOneLevel / BloonsTowerDefense

* Round 90 of [=BTD6=]. Not much to say about it except that it's the first round that the [[DemonicSpiders D.D.T.]] appears, and unlike the other intro rounds for the blimps, it doesn't come alone. The round requires that you've specifically prepared a defense capable of bypassing all three of a DDT's immunities alongside enough DPS or crowd-control to pop them before they speed through the map, otherwise they ''will'' run you down and end your run in a heartbeat.

* In late October 2023, Vortex would be put on In A Loop. While it seemed fairly easy at first, with a large cash start (1600) and allowing all Paragons to be used at once, the elite version would prove to be very difficult, with '''190% Boss speed'''. Tier 5 would prove to be a very tough challenge even with all 7 Paragons on the track.

** Bloody Puddles is an even harder remake of the aforementioned Muddy Puddles, and is generally considered the hardest map in ''[=BTD6=]'' bar none. Compared to its predecessor, it add a fifth path for bloons which, instead of using only one of the paths, now use two at the same time, one stream towards the top and one towards the bottom in a mirror fashion. As a result, on some rounds, you'll be dealing with bloons almost half a screen apart! There's also a jeep that blocks your sight, and if you pay to have it removed? [[SchmuckBait The chinook that tries to carry it crashes]], forcing you to pay for ''that'' to be removed, before you can actually get rid of it. Worse, the paths are even shorter than in Muddy Puddles, making them the tied fourth shortest paths in the game behind Blon's path and the two vertical paths of #Ouch. That map is so difficult that it took nearly five months and one nerf (reducing the hitbox of the jeep in version 14.0) for Bloons Tower Defense veteran Jajajosh to find a way to Black Border it.

* Erosion is another difficult map. The map starts with a short line at the bottom left corner, then every 18 rounds, some part of the ice will break, causing all the land towers standing on the ice to be sold (and naturally, this will not get you the money back in CHIMPS), as well as changing the path according to the coast's location. The erosion ends at round 90, the path become very short again and the remaining land masses are even less than of a flooded Flooded Valley (which is an Expert map, but is likely easier than this Advanced map)! Since you can't build Heli Pilots or Monkey Aces on the small land at the top, you'll have to rely on other kinds of permanent DPS, mainly water towers and land towers with infinite range and a small footprint like Psi or Sniper Monkeys, while using relatively cheap temporary land towers to handle early rounds. Whether it's another true expert map is still debatable, but it's definitely harder than many of the Expert maps.

* [=#Ouch=] is, in essence, this game's answer to Main Street from [=BTD5=], but with a twist; ''there's double the lanes, all of them going directly horizontal or vertical of one another, and they alternate all the time.'' Not helping is that the center of the map has a bit of water, which you have to pay to remove... and with it, you remove ''all'' of the water on the map. For a time, it was deemed outright impossible to clear the map in [=CHIMPS=] (dubbed "[=#Couch=]" by the devs and fanbase)--it ''is'' doable, but it took multiple updates after the game released to even prove it, and it is still very much an uphill battle.
* Muddy Puddles is yet another Expert track that was thought to be impossible to beat [=CHIMPS=] (with the humorous nickname of "Cuddles" used amongst fans and the devs to describe such a feat). 4 paths, all very short, with a slight wave to them; and of course, the titular puddles, which ''do'' serve as water, but are also some nasty chokepoints. It has since been made easier and has been overshadowed by... well, you can see the next entry, but even now it is quite the formidable track.
* Bloody Puddles is an even harder remake of the aforementioned Muddy Puddles, and is generally considered the hardest map in ''[=BTD6=]'' bar none. Compared to its predecessor, it add a fifth path for bloons which, instead of using only one of the paths, now use two at the same time, one stream towards the top and one towards the bottom in a mirror fashion. As a result, on some rounds, you'll be dealing with bloons almost half a screen apart! There's also a jeep that blocks your sight, and if you pay to have it removed? [[SchmuckBait The chinook that tries to carry it crashes]], forcing you to pay for ''that'' to be removed, before you can actually get rid of it. Worse, the paths are even shorter than in Muddy Puddles, making them the tied fourth shortest paths in the game behind Blon's path and the two vertical paths of #Ouch. That map is so difficult that it took nearly five months and one nerf (reducing the hitbox of the jeep in version 14.0) for Bloons Tower Defense veteran Jajajosh to find a way to Black Border it.
* Quad is another "true Expert" map. Full of short, interconnecting paths, it's very easy to lose track of which Bloons are a second away from the exit and which have just entered. There's water that's too far away from the main track to be efficient, and no tower can efficiently hit all paths at once.
* Ravine quickly gained infamy during launch to the point where it has joined the "true Expert" map category and was even impossible to beat on [=CHIMPS=] at launch. The map starts out with two entrances that split off into four exits. However, there are numerous bridges and trees in the way that blocks the sight of towers, and there is not a lot of space to place towers. No tower is able to counter all of the lanes at once, and especially the leftmost lane is extremely short. The water on the map is basically useless as well, since the line of sight for water towers are blocked when placed there. You better have plenty of powers if you want to even think about beating even Impoppable or Half Cash on this map.
* Dark Dungeons. Four paths that are pretty much all divided up into three different sections blocked by line of sight, and none of the paths really intersect. All the paths are very short too, and there is only a small pond at the bottom right of the map. There is also an [[spoiler:EasterEgg where you can activate the statue]], but for its cost, it's mostly a UselessUsefulSpell that won't do a lot against the bloons.

* Mini-Luck has one very infamous second island, where you have to beat Karts n' Darts with every single bloon turned into a camo regrow bloon that regrows 7 times faster. Druids with Heart of Oak under a Radar Scanner village are required or else the camo regrow bloons are guaranteed to replicate uncontrollably. Some people who tossed their villages or druids overboard [[UnwinnableByDesign are forced to repeat the first island]] to have a chance here. Even with the druids, poor tornado luck can cause regrow madness anyhow.

* Bloody Puddles is an even harder remake of the aforementioned Muddy Puddles, and is generally considered the hardest map in ''[=BTD6=]'' bar none. Compared to its predecessor, it add a fifth path for bloons which, instead of using only one of the paths, now use two at the same time, one stream towards the top and one towards the bottom in a mirror fashion. As a result, on some rounds, you'll be dealing with bloons almost half a screen apart! There's also a jeep that blocks your sight, and if you pay to have it removed? [[SchmuckBait The chinook that tries to carry it crashes]], forcing you to pay for ''that'' to be removed, before you can actually get rid of it. Worse, the paths are even shorter than in Muddy Puddles; in fact, they're one of the shortest in the game. That map is so difficult that it took nearly five months and one nerf (reducing the hitbox of the jeep in version 14.0) for Bloons Tower Defense veteran Jajajosh to find a way to Black Border it.

* Rounds 63, 76, and 78 in ''[=BTD6=]'' can be these without ''heavy'' bloon popping power, because large amounts of tightly clumped ceramics appear on these levels. Specifically, 63 is three groups of regular ceramics, 76 is an even larger cluster and they're all [[HydraProblem regrow]], and 78 is two large clusters of ceramics and one is all-camo. In Alternate Bloons Rounds, round 76 is {{Inverted|Trope}} in that it contains 10 B.F.B.s instead of a regrow ceramic rush. Such is Round 63's infamy that there are two special Freeplay rounds in ''[=BTD6=]'' based on it. Round 163 changes the relatively tame-sized waves of Leads and Ceramics to immense waves of really fast Camo Regrowth Fortified Leads and Ceramics, respectively (notably, it's the only Freeplay-only round where no blimps whatsoever are present). Round 263 is very similar to Round 63 in terms on the size of each wave... except Lead Bloons are replaced with (often Fortified) [=MOABs=] and the Ceramics are replaced with Fortified [=DDTs=], which can be surprisingly difficult to deal with despite their relatively low health compared to other blimps, thanks to their huge speed and their stacked nature meaning your defense's damage output in this round greatly depends on your towers' pierce (not helped by the fact as, a late Freeplay round, all blimps have 54.5 times as much health as before Round 81).

** And if you manage to survive past ''that'', once you get past Round 250, the game stops pretending to play fair and drastically ramps up the health increase for all blimps, from the hefty but manageable 35% it has been for the previous 100 rounds to a whooping 100% health per round (those Fortified [=BADs=] that were getting increasingly more difficult to deal with are now getting 40000 more HP per round, rather than 14000).

* In ''[=BTD6=]'', Chutes is rated as an Intermediate map but is [[NonIndicativeDifficulty harder than some Advanced maps]]. It consists of two relatively short paths with only a small area of overlap. Each round, bloons alternate between paths, meaning that if you want to build towers outside of that small crossover zone, they'll only be useful half the time.

* Vortex on Quiet Street and Chutes. Awful map combinations where most towers are easily going to be stunned due to the massive stun range Vortex has while towers are grouped up due to the poor map shape, alongside having numerous support towers disabled (both disable Dart paragon to make it harder to deal with Elite tier 3, and the latter also limits how many farms and total towers you can have, weakening your farming and paragon potential). Have fun beating Elite.

* Week 134's "The Year of the Rabbit" returns with chaotic custom rounds where everything from a ceramic or above is a BFB, and everything below is a red or yellow, making the rounds generally a cash starve. Although it looks dangerous, surviving the 40s will guarantee victory, although such feat will be hard due to the limited amount of starting cash. However, when you do reach the second map, it takes place on Double HP MOABs, so all the BFBs are extremely hard to take down without relying on a full-on budget defense to beat the early game. Have fun without Monkey Knowledge.

* [[AscendedMeme Blons]], a [[EasterEgg hidden track]] available through the challenge editor. It's a slightly modified version of Slons, a joke map designed by a fan, consisting of a minuscule corner track and nothing else. While somewhat easier than it looks (most of the hardest maps are hard because of splitting/changing paths and sight-blockers, while Blons has no gimmicks), it's still the shortest map in the game and there isn't much space to place towers around the track, meaning that only specific defenses can take everything out fast enough.

* Primary Monkeys Only, being on Easy difficulty, should be easy, right? Unfortunately, Flooded Valley on Primary Only is considered the hardest map to beat on this mode without powers. Only three Primary upgrades total (Blade Maelstorm ability and a bottom path Dart Monkey on a lower rock, albeit both with great difficulty, and the MOAB Assassin Ability can also target the round 40 MOAB) can see up the map's characteristic dam, leaving players to be forced to rely on a singular HeroUnit (either Adora, who can see up the dam, or Brickell, the only naval hero) to deal literally all their damage. Prepare to leak a lot.