There is blog is to showcase the hardest tracks in the entire Bloons Tower Defense series.
Bloontonium lab[]
The Bloontonium Lab is the first EXTREME! difficulty track in BTD5: it costs 50 Monkey Money to play, it gives you a massive amount of Experience Points if you win any difficulty, & it is an obviously difficult track. It has 3 short paths that the bloons travel on, water does not exist on it, & it also leaves you with a big risk of losing 50 Monkey Money without gaining any rewards if you lose. Another factor is that saving is disabled for this track. Fast-Track Mode & sandbox are also disabled with this track. The good thing with this track though is that it has a perfect center for towers.
Monkeys VS Bloons[]
This is a track based off the pool levels of Plants VS Zombies. It has 6 short tracks, no perfect tower middle & no safe havens. This track does however have water for Monkey Buccaneers.
Triangle of Insanity[]
This map is aptly named: it lacks water, & the bloons only travel on the outer ends of the triangle, so it won't take them long to reach the ends. Like the bloontonium lab, however, it does have a perfect center for towers & after you beat this track on medium or hard (which is easier said than done), you unlock the supernateral specialty!
Down The Drain[]
This one does have water in the middle, but that doesn't make it easy. Its 4 pipe tracks are very short & start the edge of the path, ending near the 4 corners of the screen. At least this one allows for saving, fast track mode & sandbox.
Crypt Keeper[]
This track starts on a gray walkway that splits into 2. The 2 ends of the walkway lead the bloons to 2 crypts that teleport the bloons to 10 gravestones which are connected to 10 ghostly paths that lead the bloons to 2 giant gravestones. While this track might be almost impossible normally, it becomes much easier with Reverse Mode turned on because the walkway's starting entrance becomes this track's only exit.
Tar Pits[]
This track has 5 Tar Pits on it. Each pit of tar is surrounded by a short, circular path that ends where it starts. The path directions reverse every 2 rounds, making this track very confusing at first. There is a huge lake of water in the middle of the track. Strangely enough, towers can placed on the tar pits without sinking like a real life animal would in a real life tar pit.