Cross() is basically the pixel-perfect version of Enter(). It is called when one object's bounding box tries to enter another's. You could use Cross() to cause something to happen or reject someone from going over an object.
Crossed() is the counterpart of Entered(), and is called when Cross() returns true and allows the object to enter another object's bounding box. You can use this for things like buttons that are pressed when you step on them.
Does that answer your question?
There's other ways to do this, such as through Bump(), but this would work too. You'll have to discuss with the real programming gurus around here on what is better and more efficient, though.
Albro's way would work but it would do so for every mob not just players, you want something that checks if they are a player then allows/disallows entry.
While reading your code posts I actually found my own way through it. Since Albro1 makes all mobs have no density with every other mob, but GreatFisher's was good but didn't actually work for me it gave me an idea and used this instead:
Crossed() is the counterpart of Entered(), and is called when Cross() returns true and allows the object to enter another object's bounding box. You can use this for things like buttons that are pressed when you step on them.
Does that answer your question?