How can I confine gameplay to a 2D space using the 3d engine?

Questions & Answers
1

I’m looking to create a fairly simple 2D platformer akin to Super Mario Bros, Sonic, etc. However, I’d like to use a 3D engine instead of sprites with all of the gameplay occurring on a 2D plane (think Trials HD).

This is easy enough to accomplish from a controls perspective, but as I started prototyping I realized that objects that I may want to pick up are rolling on the Z-axis and out of reach. Likewise, I’m not sure how to ensure that enemy AI doesn’t try to move off the plane and become unreachable (all the while attacking me from their safe “third dimension”).

Is there a smart/easy way to do this? Would I just put up two transparent planes that kind of trap gameplay items in between so they don’t move towards or away from the camera?

2

As cdrandin said, you can block the movement on any particular axis quite easily. For example:

Vector3 movement = (whatever movement direction you want);
movement.x = 0f;   // choose whatever axis you won't need
transform.Translate(movement);

It also sounds like you want to use the physics engine for powerups and the like. Instead of transparent walls (you’d only need colliders, no need for transparent meshes), check Rigidbody.constraints and Rigidbody.freezeRotation - this should give you the functionality you need.