I am writing this tutorial in response to a post in the "How Do I.." forum. The first thing to point out is that I am not an expert, and there may be an easier way to do this, but this is how I do it.
for this tutorial I will assume that you know the basics of C2
Set up your sprites, you will need A Player, A Floor Tile, An Activation Tile and A Door
give your player platform movement, and make all other sprites solid (this is done in the sprite behaviors)
Arrange your game screen like this: