Unplaceable land

Unplaceable land is a type of space that appears in many Bloons TD games. It is a feature that had first debuted on Bloons TD 4. Unplaceable land, simply speaking, is space where no types of tower can be placed and 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 (or small/medium for Navarch) 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.

In Bloons Adventure Time TD, the ally Lady Rainicorn can be placed on unplaceable land as long as a part of her footprint overlaps with another Lady Rainicorn. This also extends the overall placeable area for all other 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.