It's not as simple as giving you a snippet of code or suggestion, you need to learn about movement and collision detection first off, as that will probably give you the best idea of how to create boundaries.
They're basically side-scrollers that push the horizon up. It's nothing spectacular. Everything has a "Footprint." You base all of your movement and movement collision (not combat related) off that footprint, and have the actual clip follow the footprint around.
When jumping, have the clip separate upwards from the footprint and use the location of the footprint to determine where the character lands, or what he hits.
Order everything's depth along the Y axis.
Voila'