Marius: Yeah, 1 map per author. We didn't put it on the rules because it's always been like that in TM² as of today.

Broadsword: We're supposed to show flatness. It's not like the height-1 MTC back in TMUFX, so it doesn't matter that the ground height of canyon is 15.

Are Tunnels Allowed?

Formula says:
Are Tunnels Allowed?

Anything that doesn't cause elevation changes is allowed

Rules says:
- the track must be built low to the ground (only 3 tiles high off the ground is allowed)

If you can build a tunnel within 3 tiles above the ground, then it's allowed.

If you want to go downwards instead, you can build on the ground and within 2 tiles below the ground.

If you do choose to go downwards, you'll have to sacrifice your ability to build in those 3 tile spaces above the ground, and use only the one on the ground.

If that doesn't make sense, I'll make a screenshot.

So, in short, the race track (not counting airtime i.e. jumps) is built within 3 levels, one of which (decide yourselves which one) is ground level.

Well said.

I already began to build, but made a canyon to place platforms at ground level
i dont know if this is allowed, saying, that you can build 2 blocks under zero level, making it necessary to lower the ground
so i built 3 blocks over the ground and one under it

If you go one tile space underground, then you can only go two tile spaces above ground.

Ground = the flat surface you see when you start the editor.

Opening up a Canyon could be defined as scenery, so it's okay to do that.

although i built a platform in it to drive on?

The platform would count as one tile space, if you drive on it of course.