Quote Originally Posted by NecroDOME
- Make 2 huge invisible wall's (cubes would do the trick) and put them to the side of your game so nothing can fall off.
- Every move-frame set the Z to zero.
#1: bad idea, a lot of contacts will get generated for no useful purpose
#2: would probably work but if you set matrix to keep Z on zero, newton will re-build some internal structs to place the body into proper internal scene graph place, which can be slow with a lot of bodies.

I would suggest writing a specialized joint that keeps bodies on ground level, or you can check out this project, they likely do what you need to keep bodies at "ground":

PPIsaac is an OOP wrapper of the Newton Game Dynamics engine and is especially designed for 2D game environments.
http://www.pyroplay.de/ppisaac.html

However for starters, a upvector joint (so bodies dont fall down) with a ground level & gravity can get you started for 2d use in no time.