Stoping the player moving the camera out side the mission area

Hi all
Very soon i will be putting a demo out of my RTS game I have been having problems building the .exe but thats not the problem at the moment. First off all when i play the game in the de-bug the player can keep no moving out side the mission area, so how do a go about stoping this. And second the units can walk more or less any where how can i stop them running up clif's or though building's.

Thanks
mat

PS: More of an level designer etc than a programer so please use simple terms. lol