RPG Game ?'s

Hello, I would just like to thank u in advance 4 reading this and I really hope u can answer my questions.

I am beginning an rpg-type game in flash 8 and I want it 2 be kind of like a levels-and-ladders game if u know what I mean (like the old donkey kong game). My questions are this: What is the actionscript to make the ground ‘solid’ to where the character can walk on it? And my next question is: How could I code it to where the character cannot go upwards until he is on top of a ladder? I would REALLY appreciate it if someone answered this! Thank you!!