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Unofficial Name The term "Unplaceable land" is only a conjectural term coined by fans and has not been confirmed by official sources (e.g. in-game, Ninja Kiwi blog). |
A Dart Monkey not able to be placed on a tar pit in the BTD5 mobile track Tar Pits
Unplaceable land is a type of space that appears in Bloons TD games. Although limits on a tower's placeability are formally defined in all BTD games, with unplaceable areas consisting of the bloon track (or non-track spaces for On-Track items), space occupied by another tower's footprint, and areas out of the game's bounds, unplaceable land itself had not appeared until its introduction in Bloons TD 4. Unplaceable land, simply speaking, is space where no types of tower can be placed, and are associated with space where the bloon track never crosses. In all BTD games from Bloons TD 6 onwards, they occasionally also serve as Line of Sight obstacles.
Placing around unplaceable land[]
There isn't an easy way to get around placing towers on top of unplaceable land directly, nor are there many towers that allow some, but not all, other towers to be placed above it.
Monkey Aces in Bloons TD 4 can also ignore land placement, but in later installments they require a landing base, so this trick can no longer be done.
Monkey Engineer's sentries in and only in Bloons Monkey City Mobile can spawn anywhere within the range of the Engineer, including unplaceable land.
The use of Carrier Flagship or Navarch of the Seas in Bloons TD 6 can allow small/medium footprint towers to be placed on its platforms to be placed along a very similar location. The stacking of Portable Lakes using an Arctic Wind Ice Monkey allow the overall placeable land to rise higher and potentially directly over unplaceable land. Special Poperations with the 
Door Gunner MK can carry certain towers with it to provide vantage points above said unplaceable locations.
In Bloons Adventure Time TD, the ally Lady Rainicorn has a placeable hitbox large enough to extend slightly beyond her tower footprint, allowing land towers to be placed on bordering unplaceable spots. Additionally, Lady can also be placed on unplaceable land as long as a part of her footprint overlaps with another Lady Rainicorn, further expanding the overall placeable area for all towers to said spots.
All flying towers are capable of being above the location of unplaceable land without themselves needing to place their landing base on there.