Yellow Bloons from BTD and the stronger Pink Bloons from BTD2 are obnoxiously fast and moderately durable, and unlike many other Demonic Spiders in the game, they come extremely early. At that stage of the game, most towers are extremely liable to miss them entirely, although they will slow down as you remove their layers.
In BTD2, it was the black bloons who fit in here- they were the only bloons to be immune to cannons, and since the strongest bloon was the rainbow bloon, cannons were used a lot. The only option against a frozen black bloon were the upgraded boomerangs, which fired in a rather strange pattern. Later installments gave the cannons shrapnel to deal with black bloons.
BTD2 introduced the Lead Bloon, which is somewhat slow but has the ability to shrug off any piercing damage. In its home game, the only tower that could destroy it was the Bomb Tower, making it incredibly difficult to take care of them since they produce Black Bloons on death, which are immune to the Bomb Tower. While they're not as bad in later games, they can still be a massive pain if they happen to be sent early.
Ceramic Bloons, introduced in BTD3. They are deceptively fast and are protected by an outer shell which allows them to absorb 10 attacks before popping. What's worse is that they can No-Sell glue, almost One-Hit Kill you if they get through, and contain two Rainbow Bloons, which are plenty annoying on their own. But the problem arises when the game decides to spam them...
BTD3 also introduced the M.O.A.B. Not much to say about them except they are slow, extremely bulky, and at the time could No-Sell any effects that could normally slow down or hinder a Bloon's progress. They contain 4 Ceramics, and if one gets through it almost guarantees a Game Over. Unlike its later added parents, the B.F.B and the Z.O.M.G, what really drives them into this territory is that they tend to be spammed.
Thought the Z.O.M.G was awful? BTD 6 introduces the mightest mook yet, the B.A.D, first arriving on round 100. It's immune to the many abilities introduced that made even Z.O.M.G easier, such as M.O.A.B Shove. It an take a real beating, and if it gets popped, you get to face two Z.O.M.Gs and two DD Ts.
From BTD5... Regenerators... Regenerators... REGENERATORS... Note that this is a "trait", which means that any bloon that isn't a MOAB-class can have it. It isn't so bad for weaker Bloons, but for stronger Bloons that have more than one children, both children will regrow back into the original Bloon if left too long. For example, if a Regrow Rainbow gets popped, you end up with two Regrow Zebra Bloons, both of which will regrow, leaving you with two Regrow Rainbow Bloons
Camo bloons in BTD5. In BTD4, Camo Bloons were a single type of bloon (like blacks and whites), and towers could accidentally hit them by targeting a bloon near them, so if one in a stream of them was popped, it could start a chain reaction. In BTD5, Camo Bloons are more like a "trait", similar to Regenerators. Every bloon they contain is a camo bloon in itself. Add that to the fact that only a few towers can spot camos, and well, prepare to lose a lot of lives.
And on round 78, you get two tightly-packed ceramic rushes containing a whopping 70+Ceramic Bloons. One of the rushes is camo! Enjoy! Thank goodness there are no camo regrow ceramics in the main game.
What's that? Playing on Hard? Well, one of the last five levels (specifically, Round 82) involves a rush of BFBs and MOABs...and it starts off with a rush of camo regen bloons topped with a train of BFB bloons. Fun!
Practically all 4th tier upgrades in BTD5 count, in various ways too. Even among them, the Temple of the Monkey God stands out. It is the upgrade for the SunGod, but it fires a single stream of sun beams instead of 3 rapid streams of sun rays. Where it gets interesting, however, is its sacrificing feature. When it is upgraded, it will sacrifice any towers in its range and absorb their powers* Its secondary powers consist of golden missiles, glue splatters, ice blasts, tornadoes and a choice between a 360 spread of darts or blades. A max Temple of the Monkey God essentially becomes a rapid-firing nuke with a concentrated explosion. Not even Z.O.M.G.s will last long against it!
Another big example is the Super-Wide Funnel upgrade for the Bloonchipper. It can destroy a MOAB after two attacks and can bring a ZOMG down to BFBs very easily.
There's a premium upgrade in the fourth and fifth games called "Double cash", which doubles how much money you begin with, and how much you get from the bloons. There's also a premium upgrade called "Healthy Bananas", which allows Banana farms to give you more lives - buying both will give you enough money to buy a banana farm from the beginning, with a little extra to buy one more cheap tower. Once the farm gets a few upgrades, it becomes ridiculously easy to rack up staggering amounts of money and lives.
In Bloons TD Battles, MOABs and BFBs become this when you play in Assault Mode. You can send a MOAB onto your opponent's track starting at round 18, which is long before they're likely to be able to deal with one, even if they have unlocked the better towers and upgrades. If they don't realize they can do the same to you (or if they don't have the money to do so)... near-instant win!
Hypersonic Tack Shooters. In the first two games, by upgrading via the tab key, you could boost upgrades way beyond their normal capability. Most of them, however, didn't work. One exception, though, was the fire rate of a Tack Shooter. Using the tab key, a player can create one that fires as fast as a Super Monkey! This was fixed by BTD 3.
While not exactly a "bug", it is possible to set up a strategy that allows players to repeatedly pop regenerator bloons (of at least black/white level) for an infinite amount of time. To do this, place a 3-0 Monkey Apprentice & 1-4 Glue Gunners in a location where the wizard can blow the bloons to the start & the gunner can slow them down. The thing is, it'll require a good defense to keep the bloons from overwhelming the player (a Ray of Doom placed so that it covers most of the exit will suffice, as will a Quadruple Spiked Mines set up if the bloons in question are rainbow bloons). Unfortunately, on the flash version, it's been nerfed as of the update that introduced 3 new maps, but it's still possible to gain unlimited money with the use of the Supply Drop ability.
The Flash-only Hypersonic Juggernauts. To pull this off, build a 0/4 dart monkey and any number of other dart monkeys (up to 9) to 2/0. Activate the ability, upgrade the 2/0's to 4/0, and enjoy the 10-second streams of giant Spike Balls of Doom!
There is another, cheaper way to double Super Monkey Fan Club firing speed (also Flash-only) that may also be a bug. Simply activate two SMFC abilities right next to each other.
The above two can be combined for even more Juggernaut insanity (max speed Juggernauts!).
The Mobile and Steam versions, instead of Hypersonic Juggernauts, have a bug where Juggernaut monkeys gain the rapid-fire darts of the supermonkey; this is the only time when it's a good idea to get the pierce upgrades for a Juggernaut, because the darts can still do a lot more damage with 4 pierce than with 1.
Memetic Badass: The Super Monkey is an elite monkey if there ever was one. He fires faster than a machine gun & can switch to laser blasts when the going gets tough...& then plasma vision when it gets even tougher. He is capable of targeting enemies from far away & can gain even more targeting range. As of the 5th Tower Defense in the Bloons series, he can turn either into a Physical God that can pummel bloons when given the right sacrifices, or a Humongous Mecha that is more than capable of holding its own against massive armies.
0/0 dart monkeys, reputed to be able to destroy anything. Explanation 0/0 dart monkeys are very weak with them being able to pop only one layer off one bloon per attack, making them only usable for the first rounds. So some players thought it was funny to empower it to cheer them up, in a similar vein to Magikarp and its Splash being treated as a Memetic Badass.
Camgrow reds. Explanation For much the same reasons as 0/0 dart monkeys. By the way, camo regrow reds cannot regrow.
This agent has since made his way to the Mobile and Steam ports of BTD5, Bloons Monkey City (mobile/Windows Desktop), and finally the mobile and Steam ports of BTD Battles. Although his pro version in BTD5 can knock bloons back with BananaPeels, some GameModders such as Vanalk and JamByte have modded the farmer in Battles so it has can actually pop bloons, and the latter mod even gives the farmer his own upgrades!
Most Annoying Sound: The clank of a sharp object hitting a lead bloon. In large doses, it is... somewhat maddening and painful to the ears.
Whatever you do, do not set up a defense in the 2nd game consisting of the Hypersonic Tack Shooters and little to no cannons, though, unless you want to hear lead clank after lead clank.
Do not click Play on the title screen at the wrong time, or else you'll hear the monkey either falling endlessly or eating the banana rapidly, until you either turn off the sound effects or refresh the game.
In massive amounts, even the normal sound of bloons popping can start to hurt the ears, in the Flash versions of BTD 5 and its spinoffs.
That One Achievement: Steeley, an achievement on the steam and mobile version of BTD5, requires you to pop 250,000 Lead bloons - but there are no bloons that contain Lead bloons, so Leads are much rarer than any other bloon. With only 277 Leads appearing in an 85 round game, you could play a map on Hard mode over 900 times and still not have enough to get the achievement. The only easy way is relying on Random Missions or the Wizard Lord mission.
The latter method can be done with the aforementioned Regrow Farming trick.. Not only by popping TONS more leads than by playing normal maps, but also by raking in the cash because the regen farming patch in the Flash version doesn't apply here. Just make sure to get a Ray of Doom ASAP to minimize lag compred to the defense you start the farm with.
That One Level: Quite a few, as most of the Expert/Extreme difficulty ones from 4 and 5 qualify. However, we'll list the main ones.
The Bloontonium Lab has 3 very short paths that form the hazard symbol. This track also doesn't have any water on it, so Monkey Buccaneers are out of the question unless you get a Portable Lake.
Monkey Vs. Bloons is a remake of the pool levels from Plants vs. Zombies. As a result, it has 6 paths that go straight to the house without any bending or turning except at the end. At least this track has water for Monkey Buccaneers.
The Triangle of Insanity takes the form of Penrose Triangle. It has 3 entrances that double as exits.
Another example from 4 would be Ant Hill track. It has one entrance, but the bloons go around a loop near that entrance before heading down one of 4 separate exit paths. It has 2 puddles of water, but they only go between the 1st, 2nd and 3rd paths, none between the 3rd and 4th paths.
Clock's an extremely awkward track to play on, as the path changes as the clock's hands move, which changes every round.
Special mention goes to the downright sadistic Down The Drain (with four short paths). And god forbid you play on Impoppable Difficulty... The only thing that can save it is the fact you can do Save Scumming because it's an Expert track, rather then Extreme.
Wish to achieve 100% Completion or just earn many Awesome Points? Get ready to cry; Tar Pits has five separate paths, and the bloons will reverse direction every 2 rounds. To boot, it's also Extreme, so you can't even save the game at all. It's hard enough, the developers said the map is impossible without agents, premiums, specialities, or free towers.
Crypt Keeper has 1 enterance... But by the end, it splits into 10 separate exits to defend, the most exits seen in the series. Very arguable, but this can be Inverted if you play in reverse mode, where you have 10 entrances and 1 exit.
Yet again, the Mobile/Steam version, with Double Double Cross. It has FOUR entrances and exits, each being very short. To rub salt in the wounds, The entrances are also the exits, so don't even think of using Spike factories. It's not the hardest expert map, but it definitely stands out as one of the worst.
There are many other levels that could count, but the ones above are probably the most obvious examples.
In terms of in-game rounds rather than just maps, rounds 63, 76, and 78 can be these without heavy bloon popping power, because tightly clumped ceramics appear on these levels.
Specifically, 63 is three groups of regular ceramics. 76 is an even larger cluster, and they're allREGROW! 78 is 2 large clusters of ceramics, and one is all-camo! Yeah.
What Do You Mean, It's Not for Kids?: In BTD1 after round 13, the comment the round gives is "The super monkey tower is not a joke, he really kicks ass!". BTD5 did similar, but less severe, after round 62 using "Next level will be @#$*% hard.", but it got removed later.
Averted on the Cool Math Games version of the first game, and as of BTD2 and beyond... Save for Dummied Out text in the Flash version for BTD3, where data for placeholder text for tower descriptions does indeed use a swear, as can be seen here.note "This is a description of the tower and what it does, it also has info about upgrades and shit."