Bombardment

Bombardment is a special large track...

The opening animation
First the game shows you a high-tech-looking yet a fairly simple track, then what seems like 4 massive bombs drop out of nowhere and punch 4 big holes in the map, the now broken track then starts fixing itself in such a way that it becomes considerably shorter than it was, then in the craters you can see a 2nd track!

How it works
The only parts of this lower track you can see are what the holes have revealed, towers near to the hole (on the top layer) can fire down to bloons, and things like mortars on the bottom layer can fire to the top layer, so you've got a large map to worry about and 2 paths whilst you can hardly see one of them (although the 4 craters are quite a decent size so you can see a fair amount of the bottom layer).

And because of the technology aspect of the top layer, useful bloon properties are more common.

On the bottom layer for whatever reason there is a 5% increase to the health and speed of things down there.