Here is my "port" of scidave's tutorial "Mikey's adventure" part 2 to Construct 2. It manages depth sorting based on y position.
Look at the Depth Sorting group in the Common event sheet. Note that the origin of the objects being tested is set to the bottom. That way when Mikey's "feet" are above the base of a tree he moves behind the tree. If his feet are below the base of the tree he moves in front of it. Currently objects can only be moved to the front or back of zorder, so you have to be careful, for example in situations like the wall along the right side. If Mikey can get close to it, it would seem to behave very strangely.